move and finish cryochamber functionality from cryptogull

[HeladoDeBrownie]

cryptogull's cryochamber plugin requires elevated permissions, namely manage messages in every channel it needs to work in. this is a perfect candidate for moving to ereshkigal instead. it also currently only implements part of the desired functionality and requires manual intervention. ideally, some number of channels will be watched and their pins automatically managed, with one possible rule as follows:

whenever a watched channel reaches the pin limit, the least recently pinned message is copied over into the specified preservation channel for that channel, and then unpinned. multiple channels can have the same preservation channel

the manual component of the command can remain, so that spot-preservations can be done on channels where continuous preservation is not wanted, such as those that ought to have specific messages pinned at all times (a possible future innovation might be to preserve all but certain specified pins, but that is out of scope for the first implementation)