73: Make projects group specific

[mbramson]

Adds a group relation to projects so that users in groups can have projects that are private to that group.

Users can create projects as long as they are associated with at least one group. If they are not they are directed to a screen where they can either create or find a different group (I realized actually joining someone else's group is not implemented yet).

Once a project has been created, the group that it's associated with can never be changed (except with /admin).

TODO:

• [x] Setup Project form to allow a selection of the Group to be associated with a project.
  • [x] This dropdown should only list groups that the current user is associated with.
• [x] Make selection of group only available when creating a project
  • [x] for the update route
  • [x] for the edit form
• [x] Disable the ability to hit the project :create action with a group_id that the current user is not associated with. (i.e. add validation to the changeset)
• [x] Make the group required upon creation and update
• [x] If a user isn't a member of any groups tell them that they have to join/create a group in order to make a project since projects have to be associated with groups.

[mbramson]

@ericworkman take a look when you get the chance. This is feature complete now.

[mbramson]

Refactored lib/abilities to be more idiomatic, and betterer in general. This is done at this point.