Use `Gitlab::Access` to protected branch access levels.

1. It makes sense to reuse these constants since we had them duplicated
   in the previous enum implementation. This also simplifies our
   `check_access` implementation, because we can use
   `project.team.max_member_access` directly.

2. Use `accepts_nested_attributes_for` to create push/merge access
   levels. This was a bit fiddly to set up, but this simplifies our code
   by quite a large amount. We can even get rid of
   `ProtectedBranches::BaseService`.

3. Move API handling back into the API (previously in
   `ProtectedBranches::BaseService#translate_api_params`.

4. The protected branch services now return a `ProtectedBranch` rather
   than `true/false`.

5. Run `load_protected_branches` on-demand in the `create` action, to
   prevent it being called unneccessarily.

6. "Masters" is pre-selected as the default option for "Allowed to Push"
   and "Allowed to Merge".

7. These changes were based on a review from @rymai in !5081.

1 files changed, 8 insertions, 13 deletions

diff --git a/app/models/protected_branch/push_access_level.rb b/app/models/protected_branch/push_access_level.rb
index 22096b13300..1999316aa26 100644
--- a/app/models/protected_branch/push_access_level.rb
+++ b/app/models/protected_branch/push_access_level.rb
@@ -2,27 +2,22 @@ class ProtectedBranch::PushAccessLevel < ActiveRecord::Base
   belongs_to :protected_branch
   delegate :project, to: :protected_branch
 
-  enum access_level: [:masters, :developers, :no_one]
+  validates :access_level, presence: true, inclusion: { in: [Gitlab::Access::MASTER,
+                                                             Gitlab::Access::DEVELOPER,
+                                                             Gitlab::Access::NO_ACCESS] }
 
   def self.human_access_levels
     {
-      "masters" => "Masters",
-      "developers" => "Developers + Masters",
-      "no_one" => "No one"
+      Gitlab::Access::MASTER => "Masters",
+      Gitlab::Access::DEVELOPER => "Developers + Masters",
+      Gitlab::Access::NO_ACCESS => "No one"
     }.with_indifferent_access
   end
 
   def check_access(user)
-    return false if no_one?
+    return false if access_level == Gitlab::Access::NO_ACCESS
     return true if user.is_admin?
-
-    min_member_access = if masters?
-                          Gitlab::Access::MASTER
-                        elsif developers?
-                          Gitlab::Access::DEVELOPER
-                        end
-
-    project.team.max_member_access(user.id) >= min_member_access
+    project.team.max_member_access(user.id) >= access_level
   end
 
   def humanize