Shared user labels/tags and blocklists

[Sjmarf]

Related: #1085, #1606

Relies on #1313

The idea is that a user should be able to "share" their blocklist or taglist with other users via some sort of link or code. They should be able to either allow or deny other users from editing the list. Users should be able to subscribe to multiple shared lists at once.

This is potentially something that should be implemented at a general level rather than Mlem-specific. There's a Lemmy issue for this here. Having a general implementation would allow this to work across different clients. Therefore, this is potentially OOS for Mlem as a whole and it could be argued that we should wait for someone else to create a general implementation (or create a general implementation ourselves, but doing so would require a lot more time input than if we were implementing this for Mlem only).