Notebooks 5.x breaks widget calls

We have developed our own custom widget that displays a map inside of a Jupyter Notebook. As part of that widget development, we use require.js for a few things. Prior to Notebooks 5.x, this has worked as expected. However, with Notebooks 5.1/5.2, we are now seeing poorly formed calls. For example, require.js used to make a call like the following:

http://js.arcgis.com/3.17amd/dijit/form/templates/TextBox.html?client=gsrs

And this works. However, with no changes in our Notebook widget call, the call now looks like:

http://js.arcgis.com/3.17amd/dijit/form/templates/TextBox.html.js?client=gsrs

It has appended a ```.js``` to the path. Has anyone seen this before? Can any suggest why this might be happening with the newer Notebook versions?

Start by reproducing the custom widget's require.js calls in Jupyter Notebook 5.1 or 5.2 and compare them with pre-5.x behavior. Inspect the notebook frontend and require.js integration involved in loading the ArcGIS template URL. Done means determining why `.js` is appended and documenting or correcting the behavior without breaking the working URL form.

Written by the indexing model from the issue text.