add candidacy wizard: you can get a 500 error by selecting a post that's locked

[mhl]

If you're not in the "Trusted to Lock" group and use the add candidacy wizard to add a candidacy to a locked post, you can get through all the steps of the wizard before getting a 500 at the final submission. It'd be worth:

• Adding validation to AddCandidacyPickPostForm to check that the post isn't locked (or that the user is in the "Trusted to Lock" group)
• Perhaps only listing posts in the drop-down that are unlocked in the wizard (or marking them as locked)

[mhl]

Similarly, after the changes in #971 you would get a 403 error on trying to add someone in a locked constituency.

Thinking a bit more about this, my preference would be to create a custom widget for selecting posts in an election which ensures that:

• Locked constituencies have " (locked)" appended to their name in the drop-down
• If you're not in the "Trusted to Lock" group, the option elements for locked constituencies have the disabled="disabled" attribute set.

This could be used by both the wizard and SingleElectionFormView for the Javascript case.