Blocks: Improve date display for schedule & sessions across timezones

[ryelle]

This PR iterates on how dates & times are displayed in the blocks.

1. Update the schedule block to use format, which will use the client's timezone to do all session-grouping, layout, and display. This ensures the schedule is always showing the viewers timezone, which makes more sense for online and regional WordCamps.
2. Prefix the grid rules with the numeric date. This fixes the issue in Sessions in schedule block overlap when viewing from certain locations/timezones #562, which happened because the last session ended at 12:10 the following day, while the first session started at 12:10 -- this created 2 rules in the grid-template-rows for time-0010, but only the first was valid. The session spanned from that first start time to it's real start time, functionally the entire "day".
3. Add timezone strings to the sessions in the session and speaker blocks - this is more of a workaround, since these are rendered by PHP and we don't know the client timezone, but it's a small improvement.

Fixes #562, fixes #565, fixes #492.

Note: I'm not sure if we could consider this as fixing 565 - once this PR is applied, it should be the case that session times are always either your (viewer's) time, or has a timezone identifier. Now that the schedule time also has a TZ displayed, it should always be clear what timezone you're seeing.

Screenshots

Schedule block use the viewer's timezone. My browser timezone is EDT, the site's timezone is PDT. The sessions start at 8:00 am PDT. On the left is Safari, which shows production, showing the ambiguous "8:00am" start time. On the right is Firefox showing my sandbox, correctly showing "11:00am". (it's still ambiguous, but most people will assume local time. Organizers can add a note above the schedule)

Session overlapping (#562):

Sessions with timezones

How to test the changes in this Pull Request:

1. Set up sessions and a schedule, pay attention to your site's timezone
2. View the schedule from various timezones, the schedule should reflect your current TZ.
3. View the speakers & sessions blocks with dates showing, they should include the site's timezone

[iandunn]

LGTM overall 👍🏻

I think the Schedule block should have the TZID too, though; otherwise I worry the ambiguity will confuse some folks.

• folks who are out of town could setup calendar reminders before they travel, and end up putting the incorrect times, and miss sessions.
• some contributors are used to the times being local, and will assume that they still are unless they see something to indicate otherwise

I think the TZID on all the blocks should also link to something like timeanddate.com, so people can easily convert between zones when needed.

[ryelle]

Yeah, that makes sense. I've added it to the row heading, but I think putting it on the times in the session cells would be too verbose, so it looks like this now:

As for linking to a 3rd party time conversion site, I think that might also get visually overwhelming - to have so many links in the schedule. If you feel strongly about it, can you open another issue for that so we can discuss it there?

[iandunn]

putting it on the times in the session cells would be too verbose

👍🏻

linking to a 3rd party time conversion site [...] might also get visually overwhelming

👍🏻 - #698