Splittable and rotatable message logs

[tempest15]

It should be possible to designate separate message edit and delete logs. This will require a database migration and some changes to config, but should be fairly simple.

It should also be possible to set up auto-rotating message logs. quiltmc/cozy-discord has a version of this that could be referenced, but the system should be written fully for Lily. The frequency of rotation should be configurable, and should be different for edit and delete logs. This will require the manage channels permission, so that permission being enabled should be checked at multiple points throughout the process. We should not require that permission for the bot as a whole.