Change flair color of bots

It would be nice if bots (currently [Bot] Piston and [Bot] Arisa) could receive their own flair color, to make it easier to distinguish them from normal moderators.

See also

• https://mojangab.slack.com/archives/G1J24FGTH/p1587458417148900
• https://github.com/mojira/arisa-kt/issues/132
• https://mojangab.slack.com/archives/G0NM0QG14/p1573425105153900