User may join or leave many campaigns
Tweeters will see and be able to press a Join/Leave button to enroll
Agency can manage list of campaign subscribers
Fixes bugs in tests, missing should eqs
Refactor tests for campaigns
There's no /campaigns for non-agency users
Refactor campaigns#index so it can be reused by users#joined_campaigns

Replaces user.id for campaign.user_id so links on the campaigns table
point to the correct campaign.
Show campaigns joined by a tweeter
Add links to joined_campaigns and campaigns

- Campaign link in profile view depends on the user's role
- Tweeters should see a "Joined campaigns" link in navbar
Bug fix: missing enrollment.id on urls for "Leave" and "Unsubscribe"
Refactor EnrollmentsController by using load_and_authorize_resource
Add test coverage for agencies unsubscribing users

Fixes bug where enrollments#destroy will assume that the leave action
is for the current user, whereas sometimes campaign owners want to
unsubscribe a user.

We also test the case where a user which does not own a campaign, tries
to unsubscribe a user.
Merge pull request #11 from Intngbl/enrollments

Enrollments