Add "track" command for moderators

[vcarl]

"Report" is a great way for members to alert mods of something, but it's not appropriate for use by moderators due to the anonymous nature. This adds "Track", which can be used to log messages. Logged messages are useful for identifying patterns of abuse over periods of time — Discord search can be powerful here, searching the mod log channel for mentions:user#1234 can show how often someone's messages have been noted by moderators as problematic.

The bot logs their username at time of report separately from pinging them, so that's capture separately. If someone changes their nickname, that could be lost context when reviewing the report.