Replace diffDOM library with domdiff

A while ago diffDOM moved its code to use ES6
modules and started using various language
features specific to ES6. These two things
happened independently btw.

The result of this is that the version of diffDOM
suitable for our build pipeline, structured as an
immediately invoked function evocation (IIFE),
now requires polyfills of some ES6 features to
work in the older browsers we support, like IE11.

It's also worth noting that in the move to ES6
the maintainers of diffDOM have adopted a process
whereby users who need to support older browsers
now have to add polyfill code for any ES6 features
they choose to use.

This commmit proposes a move to the domdiff
library instead because:
- it runs on all javascript runtimes with no
  polyfills
- it is 2KB instead of diffDOM's 25KB

Domdiff takes a different approach to diffDOM, in
that it compares existing nodes and new nodes and
replaces the existing ones with the new ones if
there are differences. By contrast, diffDOM will
make in-place changes to nodes if there are enough
similarities. In other words, in most situations,
diffDOM won't change the node in $component
whereas domdiff will.

Because of this, I've had to change the
updateContent.js code to cache the data-key
attribute's value so we don't lose access to it by
overwrite the $component variable with a different
jQuery selection.

tests/javascripts/updateContent.test.js

@@ -31,9 +31,11 @@ beforeAll(() => {
 
   $.ajax.mockImplementation(() => jqueryAJAXReturnObj);
 
-  // because we're running in node, diffDOM executes as a module
-  // in the normal browser environment it will attach to window so we replicate that here
-  window.diffDOM = require('../../node_modules/diff-dom/diffDOM.js');
+  // because we're running in node, we can't execute the normal domdiff code like a browser because
+  // its variables (actually only one: domdiff) stay local to it
+  // in the normal browser environment it will attach to window so we replicate that here by using
+  // the CommonJS module version and assigning it to window
+  window.domdiff = require('domdiff/cjs').default;
   require('../../app/assets/javascripts/updateContent.js');
 
 });