Create separate `Titled` mixin for `title` attribute

* Previously, it was within `Slugged`, but sometimes one is unwanted
Add methods for model registration
minor workflow HF mockup fixes
Show one-time frequency even if frequency is not set
Merge pull request #1390 from reciprocity/feature/mockups-workflow-polish

minor workflow HF mockup fixes
Add missing copyright header
Add `GGRC.extensions` and use it for `infer_object_type`

* Basically, each extension with client-side content should register in
  the `GGRC.extensions` list.  Currently only `init_widgets` and
  `object_type_decision_tree` are supported methods.
Move ListLoader shorthand functions to `GGRC.MapperHelpers`

* This is cleaner and will enable reuse in extensions
Add models, migration, and settings for ggrc_workflows
Add initial javascript models, extensions, and templates
Remove cruft from previous commits
Make `ggrc_workflows.settings.development` a default module

* Add `ggrc_workflows.settings.development` to `GGRC_SETTINGS_MODULE`
  in `init_env` and default deployment settings
fixing minor issues for workflow mockup modal and tree items view
Merge pull request #1392 from reciprocity/feature/mockups-workflow-polish

fixing minor issues for workflow mockup modal and tree items view
adding email notifications to workflow mockup
Don't close menu on checkbox click
Merge pull request #1394 from reciprocity/feature/mockups-workflow-polish

Feature/mockups workflow polish
workflow manager mockup email notification for user dropdown
Validation for task groups creation
Merge pull request #1396 from reciprocity/feature/mockups-workflow-polish

Feature/mockups workflow polish