Make the MR interface more code focused

Problem/Motivation

Steps to reproduce

Compare the amount of code you can see on the MR interface vs the plain diff. Also, there's an awful lot of distracting visual clutter on the MR screen compared to the plain diff.

Proposed resolution

This would be an additional interface, probably triggerable by some account settings.

1. Allow suppressing all elements except code on the MR inteface. This still wouldn't be just a plain text, far from it.
2. Hover over a line of code inverts the line for easier readibility (also could be a preference)
3. Clicking the hover brings up a modal to add a comment
4. Drupal mandates 80 column width but these days everyone has a widescreen monitor so the right hand side is empty. This is where existing comments will be, initially invisible (another preference for initial invisibility).
5. The initial state of a comment is just the first line of it next to the line where it belongs. You can unfurl it by clicking it.
6. Somewhere on the top right corner you have a button to reveal comment threads: one button to show and unfurl the first and one button to unfurl all.
7. Each comment thread is a draggable modal so it's easy to see multiple at the same time with as much context as you need. The modal has buttons to collapse the current and unfurl the next / previous one if you only want to unfurl one at a time.

Now you have an extremely code focused MR interface.

Remaining tasks

User interface changes

API changes

Data model changes