Backport changes from gitlab-org/gitlab-ee!581

What does this MR do?

Backports changes that were made in gitlab-org/gitlab-ee!581, to avoid potential merge conflicts in the future.

What are the relevant issue numbers?

• Related to gitlab-org/gitlab-ee!581

Does this MR meet the acceptance criteria?

Tasks

• !5824 (merged) Backport changes from EE!581 to CE
  • Implementation
    • ::ProtectedBranches::CreateService.new
    • Can't remove load_protected_branches_gon_variables
    • has_many with count enforced
    • Extract from access levels
    • project.protected_branches.create(params)
    • Improve "access_levels.first"
    • Fix tests
  • Fix build
  • Assign to Douwe
  • Wait for review/merge