Render dropdown metadata earlier

Summary:
Ref T11179. One issue I'm getting with trying to turn actions into dropdowns is that we currently render this menu very late, which can cause us to try to add more metadata after we start resolving metadata. This won't work right now (and making it work seems unreasonably complicated), so stop doing it and fatal if something tries.

(This might make some things fatal but should be safe -- anything that fatals should have been broken already.)

Test Plan:
Browsed around looking for fatals, didn't see any.

(This primarily avoids a broken state / fatal in a future diff.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11179

Differential Revision: https://secure.phabricator.com/D16151