add a nomination command

[nikomatsakis]

I would like to have a command that looks like this:

@rustbot nominate <team>

*Nomination text goes here*

where team can be whichever team, but perhaps it starts as just lang. I think what I would like this command to do is:

• Check that the user has specified a team and included some non-empty nomination text. If they have not, then give them a response and direct them to a page that explains how to nominate. This probably requires some per-team configuration, though. (Or maybe we can just have a single page on forge that has links for each team.)
• Edit the OP in the same way as @rustbot note to include a link to the nomination comment. In conjunction with lang team: embed OP and contributors into the report #1343 this will make it easy to find.
• Add the label I-nominate-for-<team>, e.g., I-nominate-for-lang. Note that I am implying a change to how we tag nominated issues.
  • If that label is not available, we could change to I-nominate + T-<team>, but I'd prefer to adopt a new system for all teams at the same time.

[spastorino]

Related to https://rust-lang.zulipchat.com/#narrow/stream/224082-t-release.2Ftriagebot/topic/Last.20comment.20in.20topic