Restrict pushes / merges to a protected branch to specific people

# Relevant Issues

- Closes #674 
- Related to #179 

# Screenshots

![2016-08-04_16-19-12](/uploads/c81032fe262949f3084974c4e622e9eb/2016-08-04_16-19-12.png)
![](https://gitlab.com/gitlab-org/gitlab-ee/uploads/342f06078b07b39c2c817b12c81b30fe/Screen_Shot_2016-07-27_at_6.50.56_PM.png)
![](https://gitlab.com/gitlab-org/gitlab-ee/uploads/52fe1ae3027d491270ea4845ae7ce54b/Screen_Shot_2016-07-27_at_6.51.02_PM.png)

# Tasks

- [ ]  ee#674 !581 Restrict merges to specific people
    - [x]  Implementation
        - [x]  Model changes
            - [x]  Protected branch `has_many` access levels
        - [x]  Frontend
            - [x]  How to add new users / roles?
            - [x]  Dropdown should include users
            - [x]  Dropdown shouldn't include users / roles that have already been selected
            - [x]  Allow removing users / roles
        - [x]  Removing a user from the project should remove their access (?)
    - [x]  Test/refactor
        - [x]  Extract common from {Merge,Push}AccessLevel models
        - [x]  Clean up code that's removing users/roles that are already selected
        - [x]  AccessLevelController?
        - [x]  Fix build
        - [x]  Add more tests
    - [x]  Non `:push_code` users can't push even if added to an access level
    - [x]  Remove access levels when a user is removed from a project
    - [x]  Rebase off EE master instead of the "no one can push" feature branch
        - [x]  Fix create for roles
        - [x]  Fix update for roles
        - [x]  Fix delete
        - [x]  Fix create for users
        - [x]  Fix update for users
        - [x]  Fix defaults
    - [x]  Verify
        - [x]  API
            - [x]  For a developer user
                - [x]  When they are granted access specifically to
                    - [x]  Merge
                    - [x]  Push
            - [x]  For a reporter user
                - [x]  When they are granted access specifically to
                    - [x]  Merge
                    - [x]  Push
        - [x]  Default branch protection
    - [x]  CHANGELOG
    - [x]  Screenshots
    - [x]  Only show `:push_code` users in the dropdown
    - [x]  Send email when user is added/removed from a protected branch
    - [x]  Assign to {mini,end}boss
    - [x]  Fix build
    - [x]  Implement @dbalexandre's review comments
    - [x]  EE should _add_ to CE
    - [x]  Wait for [build](https://gitlab.com/gitlab-org/gitlab-ee/commit/61edf43d31452a0b972dc880e277c825d59f764f/builds) to pass
    - [x]  Test by hand
    - [x]  Assign to endboss
    - [x]  Create CE MR to backport changes
    - [x]  Implement @Douwe's comments
        - [x]  access_level#humanize
        - [x]  Blank line in _protected_branch.html.haml
        - [x]  move stuff into shared partials? (_protected_branch_ee.html.haml)
        - [x]  Can we move stuff into shared partials? (_create_protected_branch_ee.html.haml)
        - [x]  The CE version has a bunch of extra attributes here, do we need those?
        - [x]  We can't indent this section just in EE (protected_branches/show.html.haml)
        - [x]  In EE, try not to add stuff to existing views. (protected_branches/show.html.haml)
        - [x]  If we're gonna change multiline blocks to single-line blocks, we need to so in CE too. (factory)
        - [x]  Split up `ProtectedBranches#show`
    - [x]  Wait for [build](https://gitlab.com/gitlab-org/gitlab-ee/commit/3943254d5f92c9be520f685044d23bf75282d42a/builds)
    - [x]  Implement Douwe's suggestions
    - [x]  Add uniqueness validation
    - [x]  Wait for @alfredo's UI enhancements
    - [x]  Remove `show` page access controls
    - [x]  Add more feature specs
    - [ ]  Wait for review for @alfredo's work
    - [ ]  Wait for merge


See merge request !581