The Blurhash library used by Mastodon requires an input encoded as 24
bits raw RGB data. The conversion to raw RGB using Imagemagick did not
previously specify the desired bit depth. In some situations, this leads
Imagemagick to output in a pixel format using less bpp than expected.
This then manifested as segfaults of the Sidekiq process due to
out-of-bounds read, or potentially a (highly noisy) memory infoleak.
Fixes#19235.
* Add blurhash
* Use fallback color for spoiler when blurhash missing
* Federate the blurhash and accept it as long as it's at most 5x5
* Display unknown media attachments as blurhash placeholders
* Improve style of embed actions and spoiler button
* Change blurhash resolution from 3x3 to 4x4
* Improve dependency definitions
* Fix code style issues
