Friendly workflow for error handling in proposal submission.
Use the proposal slug to render a thanks page after a successful proposal submission.

Using a redirect_to so peeps can't accidentally resubmit a proposal. It exposes the slug, but I don't think that's a huge deal.

Still needs language, but the copy is in my email and Southwest wants $8 to use wifi on this airplane and I'm just not feeling it.
Style the thanks screen, include the proposal data.
Copypasta a bit too much to make the update method use the errors workflow like create does.
Some style and layout tweaks, because I'm bored and there's a half hour until I land.
Merge pull request #25 from jnf/jnf/friendly_errors

Jnf/friendly errors