Rich Text: Update TinyMCE component to use componentDidUpdate

[aduth]

This pull request seeks to update the TinyMCE component to avoid usage of the deprecated componentWillReceiveProps React function, which will log warnings when using the React.StrictMode component.

Implementation notes:

Since this lifecycle method operates on the bound DOM node, it is a fairly straight-forward conversion. In fact, this probably should have been implemented using componentDidUpdate all along.

There's probably some broader refactoring that could be done here to avoid the manual DOM mutations. This pull request is only meant to address the deprecated lifecycle usage.

Testing instructions:

Verify that there are no regressions in the behavior covered by this method:

• Placeholder visibility
• Styles application
• Classname application
• Aria key application

[wpscholar]

Changes look good to me.

[aduth]

Investigating a strange visual regression when creating a new paragraph...

[aduth]

Ah, it's obvious to me now why it wouldn't be working. We have shouldComponentUpdate always returning false, so while previously componentWillReceiveProps would have been sufficient for effecting changes, componentDidUpdate is not since the component will never... update.

[youknowriad]

Yes, I tried this as well, I wonder if this is one of those places where we should just rename the function to the UNSAFE version.

[ellatrix]

Let me have a look as well.

[aduth]

Superseded by #11368