Add users' usage statistics (admin) API endpoint

Allow for queries to early-exit if the requests have been aborted.

endpoint to be accessible to homeserver admins.

There was no need to lock it down to only global admins.

module.

Prepare for function to be used by multiple modules.

endpoint.

server.

statistics for a server.

Fixes https://github.com/turt2live/matrix-media-repo/issues/73

Fixes https://github.com/turt2live/matrix-media-repo/issues/318

Fixes https://github.com/turt2live/matrix-media-repo/issues/336

Fixes https://github.com/turt2live/matrix-media-repo/issues/355

Bump ajv from 6.12.0 to 6.12.6 in /tests/performance

Bumps [ajv](https://github.com/ajv-validator/ajv) from 6.12.0 to 6.12.6.
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](https://github.com/ajv-validator/ajv/compare/v6.12.0...v6.12.6

)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Fixes https://github.com/turt2live/matrix-media-repo/issues/344

Handle file not existing when trying to delete it

Add `urlPreviews.userAgent` config.

Fixes https://github.com/turt2live/matrix-media-repo/issues/353