Restore select2-ness of chapter leader dropdown

select2ified dropdowns will now get select2 javascript applied
to them when you add a .select2-dropdown class to the <select>.

Previously, they all just used their own magic ids.
Reorganize new event page

- Futz with margins and font sizes a whole lot
- Add dark grey btn-info
- Add helper text galore
- Reword +1 form text to not say RailsBridge specifically