Users who are red-green colour blind will likely have more difficulty using the diff viewer and reading change descriptions, as we use red for "line removed" and green for "line added". We should have a preference for changing the add & remove colours to another set of colours that are more distinguishable. This preference would apply to anywhere we use this visual metaphor.

A more colourblind-friendly alternative to green is blue for inserts and yellow instead of red for deletes. Because yellow is used by default for replaced lines, replacement lines will be a new shade of blue that appears as a cool grey for colourblind users.
I've added a new colorblind setting in user preferences which is saved under the user's profile's settings and added a colorblind class to be applied to the diffviewer (including the diff viewer in review comments) to enable the alternate color scheme.

Added a test under the profile tests of the accounts tests that tests whether the new setting saves to the profile correctly and that the setting is evaluated as false by default.