* Switch from unmaintained paperclip to kt-paperclip

* Drop some compatibility monkey-patches not required by kt-paperclip

* Drop media spoof check monkey-patching

It's broken with kt-paperclip and hopefully it won't be needed anymore

* Fix regression introduced by paperclip 6.1.0

* Do not rely on pathname to call FastImage

* Add test for ogg vorbis file with cover art

* Add audio/vorbis to the accepted content-types

This seems erroneous as this would be the content-type for a vorbis stream
without an ogg container, but that's what the `marcel` gem outputs, so…

* Restore missing for_as_default method

* Refactor Attachmentable concern and delay Paperclip's content-type spoof check

Check for content-type spoofing *after* setting the extension ourselves, this
fixes a regression with kt-paperclip's validations being more strict than
paperclip 6.0.0 and rejecting some Pleroma uploads because of unknown
extensions.

* Please CodeClimate

* Add audio/vorbis to the unreliable set

It doesn't correspond to a file format and thus has no extension associated.

* Fix avatar and header issues by using custom geometry detector

Revert a part of #6508. The file passed to dynamic styles method
was not actually a file, but an instance of Paperclip::Attachment,
which broke all styles by always returning {} from the method.

One problem with GIF avatars was that Paperclip::GeometryDetector
reported wrong dimensions for them, e.g. 120x120 GIF avatar would
for some reason be detected as 120x53. By writing our own geometry
parser, we can use FastImage, which also happens to be faster than
ImageMagick, to detect image dimensions, which are also correct.

Unfortunately, this PR does not implement skipping a `convert`
entirely if the dimensions are already correct, as I found no easy
way to write that behaviour into Paperclip without rewriting the
Paperclip::Thumbnail class.

* Only invoke convert if dimension or format needs to be changed