1. While creating a protected branch, you can set a single user / role
  for each setting ("Allowed to Merge", "Allowed to Push").

2. More users / roles can be set subsequently.

3. Repurposed 'users_select.js.coffee` for the needs of this page.

4. Move protected branch settings to the `show` page.

    - Too many settings on the single index page can be overwhelming. Also,
      if the number of users that can access a protected branch is large,
      the amount of space between protected branches in the table can be
      unwieldy.

    - This is the simplest design I can think of - we can use this
      until we have someone from the frontend/ux team take a look at
      this.

    - Move protected branches javascript under a `protected_branches`
      directory.

    - The dropdowns don't show access levels / users that have already been
      selected.

    - Allow deleting access levels using two new access level controllers.