[imp] Proposal: change Important flag to Flag for followup

[Fábio Almeida]

I believe flagging a message as important does its job pretty nicely
already. If the user is going to post a follow up, read it later, flag it so
they won't lose it or just following any crazy criteria is up to themselves.
An option to specify what the flag means would be really interesting
thought. On second thought, it could confuse less experienced users. Just my
2 cents.