Validate branch-names and references in WebUI, API

app/controllers/projects/branches_controller.rb

app/services/create_branch_service.rb

app/services/delete_branch_service.rb

app/views/projects/branches/new.html.haml

doc/api/branches.md

features/project/commits/branches.feature

features/steps/project/browse_branches.rb

lib/api/branches.rb

lib/gitlab/git_ref_validator.rb

spec/lib/git_ref_validator_spec.rb

spec/requests/api/branches_spec.rb