Allow certain Competition Form fields to be edited after announcement

[gregorbg]

Basically works as intended per the field list WCAT sent to us.

~In the backend, we have to give "normal" users (i.e. Delegates, Organizers) permissions to make these edits across the board. There is no (easy) way to restrict edits to only certain fields in the backend. Let's see which tests explode because of this assumption.~ EDIT: I used a hash_diff gem to easily compute what has been edited and then compare these edits against a list of allowed edits.

If anyone has any smart suggestions about the overrideEnabled name, I'm all ears.