add some mechanism for verifying user accounts as being particular candidates

[mhl]

Migrated from https://github.com/mysociety/popit/issues/656, essentially.

Lots of the things we might want to be able with the site would depend on having some mechanism for allowing user accounts to be verified as being particular candidates. Like Twitter's "verified" tick, this would probably involve a manual step of phoning up a candidate or their agent and checking that a particular user account on the site can make definitive statements about that candidate.

Some examples of what we'd be able to do then include:

• Allowing that user account to lock fields of their own record
• Mark particular fields as having been edited by the candidate themself
• #219
• #220

[JoeMitchell]

Chatting with Google, in the French elections they are allowing candidates to upload their own information (and change it whenever they want) — and they whitelist the domains/emails (?) in order to do this. (So a candidate('s team) sees a 'Change this information' button in the knowledge graph, that normal folks don't see — I think)