Adds other booking systems beyond eventbrite

To ease managing them and simplify the content on `index.md` they've
been moved to includes like the installation steps.

_includes/booking/eventbrite.html

@@ -0,0 +1,19 @@
+{% comment %}
+EVENTBRITE
+
+This block includes the Eventbrite registration widget if
+'eventbrite' has been set in the header.  You can delete it if you
+are not using Eventbrite, or leave it in, since it will not be
+displayed if the 'eventbrite' field in the header is not set.
+{% endcomment %}
+{% if page.eventbrite %}
+<strong>Some adblockers block the registration window. If you do not see the
+  registration box below, please check your adblocker settings.</strong>
+<iframe
+  src="https://www.eventbrite.com/tickets-external?eid={{page.eventbrite}}&ref=etckt"
+  frameborder="0"
+  width="100%"
+  height="280px"
+  scrolling="auto">
+</iframe>
+{% endif %}

_includes/booking/pretix.html

@@ -0,0 +1,20 @@
+{% comment %}
+PRETIX.eu
+
+This block includes the pretix registration widget if
+'pretix' has been set in the header.  You can delete it if you
+are not using Pretix, or leave it in, since it will not be
+displayed if the 'pretix' field in the header is not set.
+{% endcomment %}
+{% if page.pretix %}
+<strong>Some adblockers block the registration window. If you do not see the
+  registration box below, please check your adblocker settings or visit  <a href="https://pretix.eu/{{page.pretix}}/" target="_blank">the booking website</a> directly.</strong>
+<div class="pretix-widget-compat" event="https://pretix.eu/{{page.pretix}}/" single-item-select="button"></div>
+<noscript>
+   <div class="pretix-widget">
+        <div class="pretix-widget-info-message">
+                JavaScript is disabled in your browser. To access our ticket shop without JavaScript, please <a target="_blank" rel="noopener" href="https://pretix.eu/{{page.pretix}}/">click here</a>.
+                </div>
+    </div>
+</noscript>
+{% endif %}

_includes/booking/tickettailor.html

@@ -0,0 +1,21 @@
+{% comment %}
+TICKETTAILOR
+
+This block includes the Tickettailor registration widget if
+'eventbrite' has been set in the header.  You can delete it if you
+are not using Eventbrite, or leave it in, since it will not be
+displayed if the 'eventbrite' field in the header is not set.
+{% endcomment %}
+{% if page.tickettailor %}
+<strong>Some adblockers block the registration window. If you do not see the
+  registration box below, please check your adblocker settings.</strong>
+<iframe
+  src="https://www.tickettailor.com/checkout/view-event/id/{{page.tickettailor}}/"
+  frameborder="0"
+  width="100%"
+  height="380px"
+  scrolling="auto">
+</iframe>
+{% endif %}
+
+{% comment %}

_layouts/workshop.html

@@ -53,6 +53,11 @@
     <link rel="stylesheet" type="text/css" href="{{ relative_root_path }}/assets/css/bootstrap-theme.css" />
     <link rel="stylesheet" type="text/css" href="{{ relative_root_path }}/assets/css/lesson.css" />
 
+    {% if page.pretix %}
+    <link rel="stylesheet" type="text/css" href="https://pretix.eu/{{page.pretix}}/widget/v1.css" crossorigin>
+    <script type="text/javascript" src="https://pretix.eu/widget/v1.en.js" async crossorigin></script>
+    {% endif %}
+
     {% include favicons.html %}
 
     <!-- HTML5 shim and Respond.js for IE8 support of HTML5 elements and media queries -->

index.md

@@ -18,6 +18,8 @@ helper: ["helper one", "helper two"]     # boxed, comma-separated list of helper
 email: ["first@example.org","second@example.org"]    # boxed, comma-separated list of contact email addresses for the host, lead instructor, or whoever else is handling questions, like ["marlyn.wescoff@example.org", "fran.bilas@example.org", "ruth.lichterman@example.org"]
 collaborative_notes:  # optional: URL for the workshop collaborative notes, e.g. an Etherpad or Google Docs document (e.g., https://pad.carpentries.org/2015-01-01-euphoria)
 eventbrite:           # optional: alphanumeric key for Eventbrite registration, e.g., "1234567890AB" (if Eventbrite is being used)
+tickettailor:         # Optional: url bit that points to tickettailor event "1234567/abc/1100"
+pretix:               # Optional: url bit that points to pretix event "organisation/eventid"
 what3words:           # optional: what3words (https://what3words.com) address of the workshop venue, without leading slashes e.g. "globe.lessening.computers"
 ---
 
@@ -83,6 +85,18 @@ It looks like you are setting up a website for a Software Carpentry curriculum b
 {% endunless %}
 {% endif %}
 
+{% comment %}
+Various booking systems are available. They are shown in the config above, if they are empty none will show below.
+{% endcomment %}
+{% if page.eventbrite %}
+{% include booking/eventbrite.html %}
+{% elsif page.tickettailor %}
+{% include booking/tickettailor.html %}
+{% elsif page.pretix %}
+{% include booking/pretix.html %}
+{% endif %}
+
+
 {% comment %}
 EVENTBRITE