Improve `MANUAL_MODERATION_WARNING_MESSAGE_LOCATION` validation

[MattyTheHacker]

Currently we do not do any validation on input for this variable. A regex or other checks should be implemented to ensure that the channel name is valid. This will be tricky as it doesn't seem like discord have clear docs on what is considered valid.

Additionally, it's theoretically possible that a discord channel with the name "DM" exists, which could conflict with the default DM option. A note added to the documentation would likely be useful just to document this.

[MattyTheHacker]

This is also the reason it currently isn't possible to achieve 100% test coverage of config.py

[CarrotManMatt]

Perhaps the config value could be required to be prefixed by a # for all channel names, and the only valid value without a # prefix would be DM, that would differentiate it from #DM. Although on the other hand, this would be a departure from every other place we use channel & role names where we don't use the # or @ prefixes in the config values.

[MattyTheHacker]

yeah I don't think it warrants a functionality change, maybe just a note in the example env and / or the docs somewhere to say that you can't use a channel called DM cos it'll use DMs