Add hidden tag to search extension

This was a suggestion from @mattwire that we hide the extension from the UI until the styling dependency is resovled.

It seems reasonable to me as this is primarily in the civicrm-core repo for code management purposes and
making it too easy to enable before the styling is resolved could lead to confusion.

I would quite like to enable on buildkit builds just for dev visibility - but that is something to think
about

ext/search/info.xml

@@ -13,6 +13,9 @@
   </urls>
   <releaseDate>2020-07-08</releaseDate>
   <version>1.0</version>
+  <tags>
+    <tag>mgmt:hidden</tag>
+  </tags>
   <develStage>stable</develStage>
   <compatibility>
     <ver>5.28</ver>