"Message compare" should be a real diff

[thomasnordquist]

Currently if someone clicks on a message in the message history, a crude side-by-side view of both values is arranged.

We should have a real diff instead. The view should expand to the left over the boundaries of the sidebar when showing a diff.
It should not be a "line-diff" see github text-diff.

Check (br)ace and monaco diff capabilities.

[thomasnordquist]

It's now a real diff and can be disabled.