Fix VerbNet export tests
Reject parse tree when an operator is missing
Ensure added verbs won't get removed on update
Specify admin search fields in syntacticframes
Allow LevinClass to be 'REMOVED' in VerbeNet

Previously, we only stored a boolean: is_translated. Now,
translation_status can hold three different values: translated, in
progress, or 'removed'.
Support hiding Levin classes