Add a mechanism for creating events

[abibroom]

Moved from https://github.com/mysociety/sysadmin/issues/748 originally created by @davewhiteland.

An event (like a conference mySociety may/may not be represented at) should be:

• in a calendar somewhere
• have a wiki page so before, during, and after there is a findable place to record notes about it

The wiki page is the solution here because we need to be able to link to supporting documents (proposals are often in Google Drive, right?), email threads in our groups, official websites.

Look: here's the category in the Wiki -- we should be autopopulating events from a (simple?) form.

Minimum Viable Product:

• a form for creating events
• autogeneration of Wiki page

Really ought-to-haves:

• a better summary than wikimedia's well-meaning [Events:Category] currently generates
• autocreation of an entry in the Right Calendar when a new event is created; link back to wiki

Fancy pantsy

• a sensible way of coping/synching if dates/details change, but honestly we can cope without
• internal dashboard widget showing forthcoming events
• automatic notifications when events we're involved in get media mentions
• and countless other things if we actually had event data as data (yes, yes, budget limitations, I know, but this is fancy pantsy)

Note: might be good idea to have YYYY-MM-DD at start of wiki page titles so Wiki sorts things properly, I dunno. Seems feasible if they're being generated automatically and in theory could be stripped out by anything that's processing it. Implementation detail, whatever.