store election metadata at the PopIt / Popolo level

[mhl]

Currently this is stored in election-specific Django settings, but @tmtmtmtm made the following suggestion on #354

Would it make sense to have this at the PopIt/Popolo level? For example, these could be Events connected to[1] the Organisation and Term for which the election is happening.

This would then also make this data re-usable, from the same source, in consuming apps, rather than them needing to duplicate it themselves (this becomes more valuable when there are multiple parallel elections, rather than just a single general election).

[1] Until PopIt natively supports Events (mysociety/popit-api#142) these could simply be embedded on the Organization, similarly to how EveryPolitician is using legislative periods (see, for example https://inatsisartut.popit.mysociety.org/api/v0.1/organizations/legislature)

[mhl]

Election metadata is in the database now, but not exposed as Popolo Events - I've created a new issue for Popolo JSON export for the remaining things to do here.