Add ability to hide a spam issue or snippet note

 damspam hide-note <url>

This has the same 6-month account age safety check so we don't accidentally lock out real accounts. In real-world scenarios the --requesting-user-id should be supplied so we only let non-external users mark issue notes as spam. This is still a lower threshold than the issue labels we use for removing whole issues so an extra safety feature (for us): the actual spam content is string-reversed and hidden as comment in the file (i.e. between ) so we can relatively quickly restore it if the bot goes haywire.

Fixes #9 (closed) - at least the damspam part, still needs to be called by hookiedookie