Prepare frontend for handling managing protected branches custom ability

...ts/javascripts/pages/projects/settings/repository/form.js

.../javascripts/projects/settings/api/access_dropdown_api.js

...ascripts/projects/settings/components/access_dropdown.vue

...javascripts/protected_branches/protected_branch_create.js

app/controllers/projects/protected_refs_controller.rb

app/helpers/protected_branches_helper.rb

app/policies/project_policy.rb

app/views/projects/settings/repository/show.html.haml

...ascripts/pages/projects/settings/repository/show/index.js

...rojects/settings/repository/_protected_branches.html.haml

locale/gitlab.pot

spec/features/protected_branches_spec.rb

...tend/projects/settings/components/access_dropdown_spec.js

...shared_contexts/policies/project_policy_shared_context.rb

...es/features/project/protected_branches_shared_examples.rb