Add event_type field to Events, and render tag in Event cards

[csaltachin]

Addresses #2361. This PR:

• adds a new event_type field to the types t and RecurringEvent.t (and their respective metadata types) in tool/ood-gen/lib/event.ml,
• updates the Event module signature in src/ocamlorg_data/data.mli,
• adds an event_type value to the entries in data/events/recurring.yml and the data/events/*.md files, and
• makes a first attempt at rendering a tag in Event cards (for both upcoming and recurring events) on the Events page:
  (expand image)

  

Some notes:

• Entries in recurring.yml must always specify an event_type value. *.md files can omit them, but only if they have a linked recurring event.
• An upcoming event's event_type value is populated from either its *.md metadata, or its linked recurring event (if there is one). If these two sources specify different event types, or if neither specifies one, it is an error (and an exception is raised in tool/ood-gen/lib/event.ml when that happens).
• The event type tags in src/ocamlorg_frontend/pages/events.eml use the tag colors from the Community pages design draft.
• Do we also want to render these tags in other pages, e.g. the Upcoming Events section of the Community landing page?

[sabine]

Hey @csaltachin, this is awesome! Literally no suggestions for improvements here. ✨ ❤️

On the question of whether we want to render the tags on other pages as well that display events: Yes, I think we do want to consolidate the event card to be a component that is used in different places. It can be a different PR because I am merging this here just now!