Update CKEditor

[ChristopherChudzicki]

We use CKEditor5 version 31. The current version is 35. We should update.

Potenital benefits:

• A variety of new features and bugfixes have been incorporated since v31. CKeditor5 Changlog

  One feature that may be of particular interest to us is block content in lists... CKEditor5 did not support block content inside lists (e.g., tables, images) until version 34. [See https://github.com/Implement advanced lists (document lists) ckeditor/ckeditor5#2973]

  (Version 34 also includes start indices for lists, so it would be possible to do list + paragraph + resume list. I assume that would take some effort on the Hugo side, too, though.)

• CKEditor5 is being re-written in Typescript. Right now the Type definitions we use for it are very, very incomplete and community generated. Updating gets us their real typedefs, which makes working with it easier in our own code.

Potential risks

We really have no automated tests that check our integration with CKEditor. (We have a lot of tests that check our own code up to the point where we hand data over to CKEditor; but CKEditor can't run in our Node testing environment, so we have no nothing that fully hooks into it.)

And...In order to support our Markdown conversion, we've messed a bit with CKEditor internals. I'm not 100% certain that everything we've done is part of a public API that would be mentioned in the breaking changes in their changelog. Additionally, migraitng from v31 to v35 is major version bumps.

It would be nice to add some automated testing of full CKEditor lifecycle before upgrading. See #1193