The current diff viewer does not have a column indicator that shows

the character boundary. Adding the feature will allow reviewers to see

when a line exceeds n characters. 
We added a red dashed line on the diff viewer at the specified column.

The indicator defaults to 80 characters and the user can specify a

number at the input box on top. With file changes, the indicator shows

on both side of the diff viewer and changes together. We also check

for the maximum column number based on the diff viewer's size. If the

diff viewer is shrunk beyond the indicator, the indicator will be

hidden.
We added the indicator as a div which the CSS property 

border-left: 1px dashed red. Whenever the column number is changed,

moveColumnIndicator() does the calculation and moves the indicator

to the new position with jQuery's css(). Although JS function does

the calculations everytime the position changes, in theory, a user

will not be changing the position too often.
Testing Done:
Tested following behaviours:

Set column indicator to 0 (check underflow)
Set column indicator to 999 (check overflow)
Check indicator line

Browsers tested:

Chrome (Version 87.0.4280)
Edge (Version 87.0.664.5)
Firefox (Version 83.0)
Safari (Version 14.0.1)

Tested following behaviours:

Set column indicator to 0 (check underflow)
Set column indicator to 999 (check overflow)
Check indicator line

Browsers tested:

Chrome (Version 87.0.4280)
Edge (Version 87.0.664.5)
Firefox (Version 83.0)
Safari (Version 14.0.1)