site-admin: create, update, and delete feature flags

Created by: bobheadxi

Closes https://github.com/sourcegraph/sourcegraph/issues/32232 . Places feature flag configuration details in a separate page to enable more flexibility for setting up making modifications to feature flags, and implements creation, update, and deletion of feature flags.

We could add additional nice loading states and stuff but I think this is good enough to start with perhaps? I'm sure I wrote a bunch of bad code here but would appreciate any commentary!

I was going to do multiple PRs but figured it might be better to do an all-in-one thing for this functionality - still reviewable commit-by-commit though! As a user, one can also simply review the gif below

Test plan

sg run web-standalone-http 

Updated from above - you can now see references as you are creating a feature flag:

More updated screenshots: https://github.com/sourcegraph/sourcegraph/pull/32246#issuecomment-1063272161