Fix validation for discussion form

Currently, when you try to create a discussion without selecting a group, you get an unhelpful page reload, with no visible errors around what happened.

Two fixes:
- Allow a flash message to show up by setting it before the page is rendered (right now, it's set afterwards, which unhelpfully renders out the flash message one pageload too late. :) )
- Use simple_form's `f.input as: :select` instead of `f.collection_select`; the former has baked-in validation error message goodness.