Allow election AreaType to be blank in forms/admin

[struan]

Even though AreaType is a ManyToMany relationship and therefore option unless blank=True is set on the field then the admin interface will insist that you associate an election with an AreaType. We might not want to do this if we are not using AreaType lookups, e.g. a simple YNR instance which has no geolocation area lookup.

Fixes #942

[mhl]

:+1: Looks good - are you planning to do a single update of the documentation after all these, or should the update be in this commit too?

[struan]

I think a single update to documentation would be preferable otherwise I suspect lots of time on rewriting the same sections for each change.