FCP can be started again after one had successfully completed

[anoadragon453]

For instance, after an FCP merge is successful and the proposal is accepted, if a user edits the latest FCP proposal comment and there's enough votes, mscbot will start FCP again. (Example).

A fix here would be to ignore edits to FCP comments if the proposed-final-comment-period label is not present.

[anoadragon453]

Another example: matrix-org/matrix-spec-proposals#4126