synonym moderation UI

[Courey]

Create a front end for synonyms moderation behind a feature flag. It should include the ability to create, delete, and update synonyms.

Acceptance criteria:

• a feature flag exists hiding this work
• a route exists where moderators can see an index of synonyms
• a route exists where moderators can create synonym groups
• a route exists for a detailed view of a synonym group where editing is possible
• font, styling, and spacing should be consistent with the rest of the website
• the data displayed for the group name should be the eventIds, not the uuids.

[Courey]

blocked by #2164