AO3-7291 Allow Wrangles to unassign multiple fandoms from themselves :)

[not-varram]

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• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number as the first thing in your pull request title (e.g. AO3-1234 Fix thing)?
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Issue

https://otwarchive.atlassian.net/browse/AO3-7291

Purpose

Adds bulk fandom unassignment from individual wrangler pages.

• On /tag_wranglers/:id, show checkboxes and an Unassign Selected Fandoms button.
• Visibility is limited to the wrangler whose page it is (and authorized admins).
• Adds a new member endpoint to remove selected WranglingAssignment rows.
• Preserves existing single-unassign behavior from the wranglers index page.

Testing Instructions

Use two existing tag wrangler accounts in the test environment:

• Account A: the wrangler whose page will be edited
• Account B: a different wrangler (for permission check)

1. Log in as Account A.
2. Go to /tag_wranglers and assign Account A to at least 3 fandoms.
3. Go to /tag_wranglers/<account_a_login>.
4. Confirm a checkbox column and Unassign Selected Fandoms button are visible.
5. Select multiple fandoms and submit.
6. Confirm flash notice: Wranglers were successfully unassigned!.
7. Confirm selected fandom assignments were removed from Account A.

Permission check:

1. Log in as Account B.
2. Visit /tag_wranglers/<account_a_login>.
3. Confirm Unassign Selected Fandoms is not visible.

Automated coverage added:

• spec/controllers/tag_wranglers_controller_spec.rb
• features/tags_and_wrangling/tag_wrangling_admin.feature (self bulk-unassign + denied for other wrangler)

Credit

varram (he/him)

[not-varram]

By the way, I can also add the check boxes to allow bulk unassign in /tag_wranglers too, Just let me know! :)