allow particular fields for particular candidates to be "locked"

[mhl]

We've had a few cases where well-meaning volunteers have tried to change fields where the candidate has wanted a particular value, and there's a risk that other people will come along and make the same change.

Similarly, some fields take a lot of research to resolve, so an administrator might want to be able to lock them.

This sort of relates to #577 in that perhaps candidates themselves should have special powers to lock data about themself...

[JoeMitchell]

Circling around to this issue from UKPGE2017 — sounds like this is an excellent idea.

[JoeMitchell]

In fact, as someone on twitter points out, it maybe useful to lock entire candidate pages (though I don't remember any vandalism of party leaders or particularly well-known candidates at PGE2015). We might want to lock the whole of Jo Cox's page, https://candidates.democracyclub.org.uk/person/4262, for example.