Adjust User group search to mimic user search

[yuenmichelle1]

Part of User Groups stats effort. See: FE Stats Project Board here: https://github.com/orgs/zooniverse/projects/36/views/2?pane=issue&itemId=41487549

Want is to mimic user_group admin search to mimic that of user search. (But in our case instead of searching by user.login we search by user_group.display_name (not a unique field)).

Compared to the last migration where we added a tsv column. https://github.com/zooniverse/panoptes/blob/master/db/migrate/archive/20151210134819_add_tsvector_to_projects.rb there are a couple differences:

• with the addition of strong_migrations gem post this ^ 2015 migration, we now have to add safety_assured on the creation of sql trigger
• I chose to separate each operation (create the column, add it as a generalized inverted index, create the sql trigger) into its own migration file.

Review checklist

• First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• Are all the changes covered by tests? Think about any possible edge cases that might be left untested.

[yuenmichelle1]

Pinging @mcbouslog so that he and I can test against zooniverse/Panoptes-Front-End#6968 once this change is merged into staging

[zwolf]

LGTM