Allow bulk edit layouts to use embedded forms

When a bulk edit layout uses a custom element(s) with embedded input widgets, the bulk edit UI becomes unresponsive and fails to save changes.

Steps to reproduce:

1. Set up a bulk edit layout and button contribution using the tutorial documentation
2. Create a new element that includes input widgets (sample used with tutorial attached).
3. Replace the corresponding widgets in the bulk edit layout with the custom element.
4. Create 2+ documents in a workspace; select 2+ and click the bulk edit button from the top toolbar.

Expected behavior: bulk-edit-widget surrounding custom element responds to changes made to its containing inputs' values. Clicking Save applies the changes to the selected documents.

Actual behavior: bulk-edit-widget surrounding custom element is unresponsive to changes. Clicking the Save button causes it to hang with a loading spinner, forcing user to click Cancel. Changes are not applied to selected documents.