Modified styling for diff viewer lines to be colorblind-friendly

Review Request #10981 — Created March 29, 2020 and updated — Latest diff uploaded

Review Board

The red, green, and yellow colors used in Review Board's diff viewer aren't
easily dintinguishable for users with various colorblind disabilities.

To address this, icons have been added to inserted, replaced, and deleted
diff viewer lines (+, ~, and - respectively) to visually represent the type
of change made without having to solely rely on color. Alignment for equal
lines (unmodified lines of code) was also updated; line numbers for all
diff types should now be aligned.

I've attached a video below showing how the diff viewer would like with my
new changes.

For more information on how I came up with these changes, see my notes here.

All current unit tests pass. No additional tests were made.