Add controls to delete all files or sever all relations with a remote instance

What does this PR do? (Please give us a brief description of what this PR does.) Adds UI to use pre-existing admin endpoints for instance moderation. Also fixes one endpoint with incorrect DI annotation. Addresses some of #636.

I am unsure whether this was a clean merge, I run a custom patchset.

Endpoints:

• admin/federation/delete-all-files
• admin/federation/remove-all-following

Contribution Guidelines By submitting this merge request, you agree to follow our Contribution Guidelines

• I agree to follow this project's Contribution Guidelines
• I have made sure to test this pull request