Adds multi-school support

  - new current_school helper in application_controller, which is set in the following order of precedence (lowest to highest):
    1. defaults to first venue's school (assumes a venue exists, which made tests a little heavier, but is a fair assumption) if none of the others match
    1. if the user is logged in and has a school set on their profile, this is used
    1. if there is a school set on the session, this is used
    1. if a school gets passed as a query param (?school=<slug>) then this has top priority
  - users can set their school when signing up, and can update their school when editing their profile
  - current users will have their school set to 1, which is the only existing school in production and makes sense
  - I didn't like creating a venue in all the acceptance tests, so there's probably a more elegant way to mock up current_school for all the acceptance tests that aren't explicitly using it
  - dynamic school dropdown in menu could probably be styled better, but this accomplishes the task
  - notifications only sent to subscribed users who attend the school where the lesson will be taught. Possibly future enhancement to let users subscribe to nearby schools as well.
Merge pull request #11 from btaitelb/multi-school

Adds multi-school support