Move election models into own app.

Somebody on the mailing list said that moving elections and polities into it's own app was a thing so I did elections but moving polities is another story since it depends on `BaseIssue` making a circular dependency if we move it into a separate app. On the other hand `BaseIssue` doesn't seem to do much other than having a description field so maybe we should just deprecate it and move polities into a separate app.