Disallow membership creation/reactivation if user_group is inactive

[yuenmichelle1]

Disallow membership creation/reactivation if User Group is Inactive. Added specs for membership reactivation which can be done via POST (use case is if a user leaves a group and uses Join Link to re-join a group)

Follow up from Slack convo found here: https://zooniverse.slack.com/archives/C010QAPB67J/p1724949294205289

Part of User_Groups Functionality Coming from new Fe-Root.

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.