Finalize forum story before 1.0

[aturon]

We've been transitioning away from the mailing list and towards http://discuss.rust-lang.org/, but this transition is not complete:

• The mailing list is still open and gets some traffic.
• The discuss forum is currently intended for "internals" discussion only, and doesn't have a good space for user discussion.
• The discuss forum categories should be revamped.

[aturon]

Nominating for 1.0 (not beta)

[aturon]

cc @brson

[sanxiyn]

I think this can be closed now.