First step of some UX improvements to events
Merge pull request #154 from creature/student-event-ux

Some copy changes for events.
sign up to receive an invitation is dup when
there are no events.

also move inside section.
Move some logic about available slots out of helper methods, and onto the Invitiation/Sessions models.
Push some more logic down to Invitations, rather than checking roles from the views.
Don't produce output from these Rake tasks if running in the test suite.
Shorter version of these if clauses.
Control output of rake tasks using test doubles rather than checking the Rails environment.
Merge pull request #155 from creature/refactoring

Move some model logic out of a presenter; tidy up test output.
Fix dead link to "How to be an effective teacher"
Merge pull request #156 from ForbesLindesay/patch-1

Fix dead link to "How to be an effective teacher"
Add counts for coach spots and attendee numbers, to make event admin easier.
Merge pull request #157 from creature/event-admin-tweaks

Add counts for coach spots and attendee numbers