Validate Channel Constants on Bot Startup

[sco1]

Related to python-discord/sir-lancebot#335, the bot's channel constants can occasionally get out of sync, resulting in #deleted-channel references in some of the error responses.

In order to alert staff that this desync has occurred, a utility function should be added to the bot's startup routine that validates the channel constants & logs a warning if any of the channels do not exist so the constants file can be updated appropriately.

[Numerlor]

I'd like to give this a try if you weren't planning for the same assignee as on the seasonalbot issue.
For the logging, should it also send a message somewhere to notify or just log as a warning/error?

[sco1]

Go for it. With Sentry now integrated, logging a warning is sufficient.