Implement WFJT prompting for limit & scm_branch

add feature to UI and awxkit

restructure some details of create_unified_job
  for workflows to allow use of char_prompts
  hidden field
avoid conflict with sliced jobs in char_prompts copy logic

update developer docs

update migration reference

bump migration

awx/ui/test/spec/workflows/workflow-add.controller-test.js

@@ -144,6 +144,8 @@ describe('Controller: WorkflowAdd', () => {
                 description: "This is a test description",
                 organization: undefined,
                 inventory: undefined,
+                limit: undefined,
+                scm_branch: undefined,
                 labels: undefined,
                 variables: undefined,
                 allow_simultaneous: undefined,