Timezone controls (optional, default turned on)

[cwant]

Summary of changes

This ports over the timezone controls from Explora to Elixir/TeSS.

Motivation and context

It's helpful to see events in your own timezone. This PR gives a toggle between showing times in the timezone of each event, and showing times in the timezone detected by the webbrowser.

The timezone control widget shows up on any page where there is an event listed (e.g. events#index, events#show, upcoming events, etc.).

The timezone controls can be opened and closed for visibility.

An added feature: the event edit page allows you to enter times in the timezone of the event (rather than having to do the mental math of entering it in GMT and hoping you've done that correctly).

Note : this uses the ffi-icu gem. For our TeSS we use this so that the timezone names appear in the language that is selected in our user interface. E.g. heure d’été du Pacifique nord-américain when French is selected instead of Pacific Daylight Time.

Screenshots

Here is the initial hit on event#index, with option "event timezones" selected by default. Two events with different timezones are shown.

Here is the same page, with the timezones controls displayed via the Change... link. (Clicking the x hides the controls again.)

Here is the page with my timezone (detected by the browser) is clicked:

Here is the editing link for start/end times, done in the timezone selected for the event (converted to GMT when saved to the database):

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.

[kennethrioja]

Many thanks @cwant for taking the time to do this PR – really appreciated and really helpful!

Before this PR, I tried on my side to cherry-pick some of your commits from your forked TeSS, so far I picked up almost the same one :

• except the last two ones you added (dca60cd and af8b11e)
• and this one I have added on my side (cwant@eceee25)

I'll test on my side if everything seems ok for me and if I don't need the last I've added on my side.

I can definitely help review this PR @fbacall – if you want to throw a copilot, please do so.