SCM Inventory model, view, and task system changes

Inventory source file-type combined with a linked project
will allow the inventory source to be updated when the
project is updated. The inventory update runs in the
post-run hook of the project update.

docs/CHANGELOG.md

@@ -12,3 +12,5 @@
   group serializer and added `user_capabilities` to inventory source
   serializer, allow user creation and naming of inventory sources
   [[#5741](https://github.com/ansible/ansible-tower/issues/5741)]
+* support sourcing inventory from a file inside of a project's source
+  tree [[#2477](https://github.com/ansible/ansible-tower/issues/2477)]

docs/scm_file_inventory.md

@@ -0,0 +1,63 @@
+# SCM Flat File Inventory
+
+Users can create inventory sources that use content in the source tree of
+a project as an Ansible inventory file.
+
+## Usage Details
+
+Fields that should be specified on creation of SCM inventory source:
+
+ - `scm_project` - project to use
+ - `source_path` - relative path inside of the project indicating a
+   directory or a file, if left blank, "" is still a relative path
+   indicating the root directory of the project
+
+A user should not be able to update this inventory source via through
+the endpoint `/inventory_sources/N/update/`. Instead, they should update
+the linked project.
+
+An update of the project automatically triggers an inventory update within
+the proper context. An update _of the project_ is scheduled immediately
+after creation of the inventory source.
+
+The project should show a listing of suggested inventory locations, at the
+endpoint `/projects/N/inventories/`, but this is not a comprehensive list of
+all paths that could be used as an Ansible inventory because of the wide
+range of inclusion criteria. The list will also max out at 50 entries.
+The user should be allowed to specify a location manually in the UI.
+This listing should be refreshed to latest SCM info on a project update.
+If no inventory sources use a project as an SCM inventory source, then
+the inventory listing may not be refreshed on update.
+
+### Still-to-come 3.2 Changes
+
+As a part of a different feature, it is planned to have all inventory sources
+inside of an inventory all update with a single button click. When this
+happens for an inventory containing an SCM inventory source, it should
+update the project.
+
+## Supported File Syntax
+
+> Any Inventory Ansible supports should be supported by this feature
+
+This statement is the overall goal and should hold true absolutely for
+Ansible version 2.4 and beyond due to the use of `ansible-inventory`.
+Versions of Ansible before that may not support all valid inventory syntax
+because the internal mechanism is different.
+
+Documentation should reflect the limitations of inventory file syntax
+support in old Ansible versions.
+
+# Acceptance Criteria Notes
+
+Some test scenarios to look at:
+ - Obviously use a git repo with examples of host patterns, etc.
+ - Test multiple inventories that use the same project, pointing to different
+   files / directories inside of the project
+ - Feature works correctly even if project doesn't have any playbook files
+ - File related errors should surface as inventory import failures
+   + missing file
+   + invalid syntax in file
+ - If the project SCM update encounters errors, it should not run the
+   inventory updates
+