Check votes are valid.

[Tom-Johnston]

Votes of the wrong form can cause the JavaScript to get stuck in an infinite loop and crash. e.g.

• AV (and presumably the other runoff voting methods) loop forever if the only films to remove are not in the listOfCandidates.
• Borda count can't cope if a film isn't in a vote.

We should sanitise the votes either on the server, in the Javascript voting methods or both.

Fortunately the members are too nice and/or too incompetent to ruin film night by submitting votes of the wrong form. Unfortunately they have almost managed to ruin film night by submitting terrible films.

[jstockwin]

I have done this server side in 567bb2252c802b061adb2c315ba6504439d0c244

Assigning to @Tom-Johnston for JavaScript sanitation. If you don't feel this is necessary, then feel free to close this issue.