Add moderation logging

[dawnofmidnight]

(part of the Carl ⇒ Ferris migration)

We'd like to have support in Ferris for moderation logging. This falls roughly into a few categories, each of which should have support for logging their information into their own channels (specified as env vars with ID).

• #rustlog-user: server joins and leaves, role adds/removals, profile changes
• #rustlog-mod: moderation actions (both by moderation commands and discord UI) (see Moderation suite #51)
• #rustlog-message: message edits and deletions
• #rustlog-server: changes to channels, server icon (already done by Ferris)
• #rustlog-voice: VC joins and leaves

This could be implemented as multiple PRs, but I suspect the functionality is similar enough that a single one with them all wouldn't be too bad.