Simplify DocumentationInputFilter

- Extend InputFilter, instead of implement InputFilterInterface; ensures
  it can be used easily with InputFilterManager, and that interactions
  match those of other input filters (e.g., can only pass arrays or
  traversable objects for data).
- Make unit tests consistent with RPC/REST; name all data provider data
  sets.
Added ag-form awareness to documentation screens

- Added ag-form to each of REST and RPC documentation-edit templates
- Added agFormHandler to documentation controller, incorporated into save action
- Updated api.js to return data from the documentation/input-filter save
  actions