Allow to create new branch and empty WIP merge request from issue page

app/assets/javascripts/create_merge_request_dropdown.js

app/assets/javascripts/issue.js

app/assets/stylesheets/pages/issues.scss

app/controllers/projects/branches_controller.rb

app/controllers/projects/issues_controller.rb

app/models/issue.rb

app/serializers/merge_request_create_entity.rb

app/serializers/merge_request_create_serializer.rb

app/services/merge_requests/create_from_issue_service.rb

app/views/projects/issues/_new_branch.html.haml

app/views/projects/issues/show.html.haml

...gs/unreleased/28558-create-new-branch-from-issue-page.yml

config/routes/project.rb

spec/controllers/projects/branches_controller_spec.rb

spec/controllers/projects/issues_controller_spec.rb

spec/features/issues/create_branch_merge_request_spec.rb

spec/fixtures/api/schemas/branch.json

spec/fixtures/api/schemas/merge_request.json

spec/javascripts/issue_spec.js

spec/models/issue_spec.rb