Allow marking with Staff Colour in the compose menu

[bts (Brendan Schlagel)]

@debryc

Sometimes we need to have a meta conversation, though, and so we use the staff color to differentiate between when we’re posting as users and when we’re posting as staff commenting on the behavior of a user.

Do the meta conversations need to be public? If not I think replying with “whispers” could work well for this, basically exactly what it’s intended for!