Add setting that allows admins to choose which Git access protocols are enabled

CHANGELOG

app/assets/stylesheets/framework/buttons.scss

app/controllers/admin/application_settings_controller.rb

app/helpers/application_settings_helper.rb

app/helpers/projects_helper.rb

app/models/application_setting.rb

app/views/admin/application_settings/_form.html.haml

app/views/shared/_clone_panel.html.haml

...d_enabled_git_access_protocols_to_application_settings.rb

db/schema.rb

doc/api/settings.md

...features/admin/admin_disables_git_access_protocol_spec.rb