Convert mixins to controllers

[spelkey-ucd]

Lit 2.0 has a new "controller" feature, which has a lot of overlap with mixins, but in many cases it's more appropriate to use a controller.

From Lit docs:

In general, if you're trying to decide whether to package a feature as a controller or a mixin, you should choose a controller unless the feature requires one of the following:
-Adding public API to the component.
-Very granular access to the component lifecycle.