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.