Most common filesystems limit paths to 255 bytes.
This change brings down the media ID length to be similar to
Synapse servers (25), and makes it possible for clients to
download media with the ID included in the filename.

They are not in the spec and maubot relies on that

The 404 error for /profile in the spec says "There is no profile
information for this user or this user does not exist.", but Element
assumes every 404 is a user that does not exist.

Replace None with Some("") for state_key

The only situation that isn't working yet is sending `left` events for
users when the sender leaves the room

This fixes the panic from ruma "index out of bounds"

Move all state event appending to append_state_pdu.

feat: first steps towards joining rooms over federation
Add state-res as a dependency of conduit
Add reverse_topological_power_sort before append_pdu
Implement statehashstatid_pduid tree for keeping track of state
Clean up implementation of state_hash as key for tracking state