Undo broken in collaborative editing session

[mejo-]

Describe the bug

In collaborative editing sessions with more than one client, undo doesn't work anymore. Undoing your own changes only works as long as no remote changes got applied to your document state.

In cases where undo doesn't work, instead the cursor jumps to the end of the document.

To Reproduce
Steps to reproduce the behavior:

1. Open a document with two sessions (can be same user)
2. Do a change in one session, use undo function, see it doing its thing
3. Do a change in one session, do a second change in second session, try undo in first session
4. Undo doesn't work as expected

[mejo-]

This commit broke undo: 2da1f89

Probably we have to make sure to always either load extension-collaboration or extension-history.