This introduces new settings in the Diff Viewer Settings page to control

whether Trojan Code confusable checks should be enabled or which

languages should be considered safe.
This is shown in-between the "Limits" and "Advanced" section. The list

of languages is presented as a series of checkboxes. There aren't too

many there, and we don't really have a better widget for displaying this

list at this time (ideally we'd use the Related Object selector, but

it's not built for this, and I wanted to minimize the UI work for this

particular change).
These settings are stored in a general way, opening the door for future

code safety checker preferences. They're available to the diff machinery

through the new DiffSettings object, and used in the chunk generator

when performing a check.
Changing these settings will automatically invalidate any caches/ETags,

ensuring they take effect immediately.

Unit tests pass.
Tested with a Trojan Code sample set, tested with confusable checks on,

on with excluded languages, and off. Verified that the caches always

reflected the new settings, and that the results were what I expected.