For Global View: Standardize name of validity status column (admin review status column).

[kfogel]

As per commit a5c7e0b and this Zulip conversation, the name of the validity status column varies between competitions. We should settle on a column name with LFC and start using it across the board.

Not only the column name, but the allowable values need to be standardize. This is a prerequisite for being able to do meaningful cross-competition comparison in Global View: if we want to ask "How far did this proposal get?" across different competitions, then the stages a proposal can go through in each competition have to be commensurate.

(Relatedly, not all competitions show invalid as well as valid proposals, but that's issue #80.)