Visual diff UI and options for toggling visual diff mode

[agjohnson]

I already found myself wanting an easy way to toggle:

• Diff on/off
• Subtractions on/off

And probably want some way to link into at least one of these, like:

• Hook into the flyout menu, or an alternative menu UI for debug level stuff. We've talked about a debug toolbar for RTD concept, but the flyout menu is already existing UI as well.
• Automatically toggle visual diff mode based on query param, for linking into the view from the dashboard or from a PR review check/etc.

We've discussed combining this with an action for posting the files that have changed in a PR.

[benjaoming]

I'm a big +1 on this, especially the query param :+1:

Adding this feature gives a lot more "acceptance for bugs". If the page diff is broken, you can switch off the diff and it won't be in the way.

[humitos]

I started this work as part of the new js client at https://github.com/readthedocs/readthedocs-client/pull/63