[Delivers #62894440] RDE for Digital Objects

Happy to review this in person when I return for Monday... but thought I'd post it up.

The backend has new endpoints for adding children to a DO and the models have included the required mixins.

There's a large bit of refactoring to allow sharing of the original RDE templates and javascript.  

I've added a frontend mixin to allow reuse of the RDE param filtering logic - this is found in the new class RecordChildren, which is included into the various '*Children' models.

PR also includes a patch for the column resizing which looks to have been broken with the recent jQuery upgrade.

I've also introduced a new UtilsController to home the shared endpoint 'generate_sequence' - I couldn't find another logical place for it.