Define modes of operation and what we can/cant do in each mode

This is inspired slightly by CanCan, but I didn't really want to introduce a new dependency.  Let's see how it goes.
Add Vestibule.mode_of_operation and set it from ENV in an initializer
Add a controller helper_method for can?
Outline what we want to happen on the proposals controller
Do the easy stuff, don't show mode-dependant call to actions.
Fix the proposals controller for mode dependant operation
Make suggestions mode-dependant
Change i_am_alerted to cope with multiple alerts on the page

Capybara's within will pick the first instance of whatever selector you give it.  If that doesn't happen to be the one that has the content you are checking for, oh well, sucks to be you.
Make sure we are in a known mode at the start of each integration test.
Don't show the 'propose (another) talk' CTA on the dashboard if we can't create proposals.
Add a global banner if we are in archive mode.
Re-enable and make mode-dependant the selections & agenda stuff
No need to have separate make and withdrawn rules for selection
Merge pull request #27 from rubymanor/modal-operation

Modal operation