React to maintainers mentioning the bot

[simonbasle]

This would provide an alternative path for fast-tracking a PR:

• user X mentions the bot with a keyword, like @bot fast-track blah blah blah, in a PR comment
• the bot checks X is a maintainer
• if so, the bot will:
  • put the watchedLabel on the PR
  • do an approve review, mirroring the blah blah blah description
  • send a similar message in slack (again mirroring the description)

⚠️ The bot MUST ignore labeled events that have the bot as sender ⚠️

[simonbasle]

This has been implemented in 99f77cd and dismissal by deletion of comment in 969cbdf
The bot doesn't put the label yet though... Keeping this open for the later