Add ability to add an admin to a location

This commit also includes other fixes and enhancements such as:
- remove the short_description requirement
- remove the maxlength constraint for short_description
- refactor authorization methods
- redirect to "/locations" with error message if new location can't be created
- fix "new" action so that it checks for presence of locations and org
- send user directly to "/locations" after signing in
- update seeds and factories to include a user with a generic email
- when a user with a generic email creates a new location, automatically make them an admin for that location
Merge pull request #53 from codeforamerica/add-admin-to-location

Add ability to add an admin to a location