Feature request: allow permanent caching of media from remote instances #249
Description
So, assume you follow someone on a remote instance for a while.
Then that instance announces that it's going to shut down permanently.
You have all the content from the user you followed from that instance cached on your instance.
Since Smithereen uses an LRU cache for caching media from remote instances, it's only a matter of time before all the media from that instance will be deleted from your cache and lost in time like tears in rain.
To prevent that, it would be useful to allow server admins to enable permanent caching of media files for certain instances (and maybe individual actors?).