Implement basic undo history

[aduth]

This pull request seeks to introduce an undo history for blocks state.

Implementation notes:

The undo implementation is roughly based on the one explored in official Redux documentation.

redux-undo is a popular alternative module which exists to implement this behavior, but also includes a number of features we don't need.

To the previous point, the changes include a proposed combineUndoableReducers which defines getters for transparent property access on the original reducer object (allow access by state.blocks.byUid directly rather than state.blocks.present.byUid). I feel ever-so-slightly uncomfortable including enhanced objects like this in state.

We may want to move these utilities to a separate file or, more likely, a separate module, but they're included here for initial discussion before being referenced more formally.

Testing instructions:

Verify that unit tests pass:

npm run test-unit

Confirm that upon changing a text block†, the undo buttons become activated in the editor header and behave as expected upon toggling.

† The text block's change behavior is currently only triggered when blurring the contenteditable, meaning you'll need to click elsewhere on the screen first before the buttons become active.

[youknowriad]

I'm personally satisfied with this implementation for now. We could revisit later.

[youknowriad]

I'm personally fine with this

[youknowriad]

I did the same simplification in #407 (We'll have some conflicts)

[youknowriad]

Maybe it's time to start splitting those reducers into separate files

[mtias]

Let's keep in mind the cognitive overload nd perceived complexity for people whenever we split files. (Part of why redux is perceived as more complex than what it is.)

[aduth]

Maybe it's time to start splitting those reducers into separate files

Is this feeling compounded by the addition of the utilities to the file? I'd like to have those moved first before anything else.

[mtias]

What's the thinking behind this?

[aduth]

What's the thinking behind this?

Memory, but in hindsight it might be a bit premature. We can remove it.

[aduth]

• Rebased to resolve conflicts
• Removed options.limit from undoable
• Moved reducer enhancer to utils/undoable-reducer.js

[youknowriad]

LGTM 👍

Later, we'll probably need to keep track of the focus config in the state. This would allow the undo/redo to move the cursor to the right position