Improve events aggregation

[sbesson]

At the moment, the requirement to update manually the events/index.html page for everyt new event added is quite tedious.

It would be useful to rethink the current strategy to simplify the discoverability into the index page. Possible options include (but might not be limited to):

• treating events as posts with a specific category, similarly to the blog migration - see Migrate OME blog content into main OME website #515
• define a new collection like minutes - see Community meeting updates #403

Originally posted by @sbesson in #536 (comment)