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

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

Fixes https://github.com/turt2live/matrix-media-repo/issues/349

Fixes https://github.com/turt2live/matrix-media-repo/issues/346

When trying to clean up database after a faulty config deleted the
existing files, I called /_matrix/media/r0/admin/purge/old but that was
failing due to the file not existing.

Code jsut swallows the error as deleting a non existing file shouldn't
be fatal

Logs-ish:

Deleting thumbnail with hash: e8ba5faa6bae71647a3a8e7e6328bf3d93343e96666eda14e5487b5e60760d80" before_ts=1632086268000 contentLength=2 contentType=application/json include_local=true isRepoAdmin=true method=POST queryString="before_ts=1632086268000&include_local=true&keep_profiles=false" resource=/_matrix/media/r0/admin/purge/old usingForwardedHost=true
Error purging media: remove /var/matrix/media/c3/43/82a1828f3222468497ac8ccc0f490b252c6a: no such file or directory"
Replying with result: *api.ErrorResponse &{Code:M_UNKNOWN Message:error purging media InternalCode:M_UNKNOWN}" contentLength=2 contentType=application/json method=POST queryString="before_ts=1632086268000&include_local=true&keep_profiles=false"  resource=/_matrix/media/r0/admin/purge/old usingForwardedHost=true

Bump lodash from 4.17.19 to 4.17.21 in /tests/performance

feat: support `org.matrix.msc2705.animated` query parameter

Mitigate media repo XSS attacks on IE11 via the non-standard X-Content-Security-Policy header.

Add support for S3 storage classes