The diff viewer settings page is one of the oldest pages in the admin

UI, and its organization left much to be desired. Some upcoming work

will be adding to this page, and as such, it's time to clean it up.
This now groups the settings into 3 fieldsets:

Appearance
Limits
Advanced

Appearance contains the checkboxes for trailing whitespace and syntax

highlighting, plus the field for mapping file extensions to

highlighters.
Limits contains the diff size limit and the max lines for syntax

highlighting.
Advanced contains everything it had before, plus the "Show all

whitespace for" field, which should generally no longer be used.
Some of the field labels and help text have been tweaked to be more

clear and to provide recommendations.

Verified the page's contents on desktop and mobile.
Checked for spelling errors.