Centralize events repo and switch to new format

[fallow64]

Adds a submodule to https://github.com/CHTC/Events with the plan to centralize all events from CHTC-ran websites to a single repo (like the current setup with articles).

The new articles have a slightly different format that should hopefully be non-Jekyll specific and overall be nicer to work with (see the CHTC/Events readme).

Sorry this PR is a bit messy. Here is a basic rundown:

• The current _events collection is renamed to _events_legacy, and the new _events collection replaces it, which is a git submodule.
• The "new" events will have the permalink of YYYY/MM/DD/event-title. The "legacy" articles will retain their current permalink.
• event-card and event-header are changed to accommodate the new format.

  • event-page and layouts/event cannot be changed because they are directly referenced by legacy events. Therefore, they keep the same legacy formatting.

    event-page will not be used by the modern events, and layouts/event_modern is created. Instead of relying on the markdown frontmatter to specify layout, for modern events, we will just use the default in _config.yml

• get/future_events.liquid and get/past_events.liquid are updated to filter according to the site key.
• events.html (the directory of all events) is updated to use the new format.

Notes:

• We could remove the legacy articles from SEO in the meta tag.
• Events that are not intended to be published on Path are included, just not listed within the event directory. Is this OK? How is this handled with articles?
• This is quite a lot of work (and adds confusion to the codebase) for only 20 links. It might just be easier to replace the legacy events with a redirect to the modern versions?
• future: true is added to the config.yml to allow for future events. This may mess with articles and have unintended consequences?
• The only events added to CHTC/Events were Path events for now.