Unitless number in CSS `calc()` causes the computation to fail

See this comment for context.

When switching from Sass computations (finalized as a single value before output) to CSS calc computations (computed by the rendering engine in the browser), we need to ensure that each number has an appropriate unit.

In many cases, this will probably be some measurement unit like px or rem, but it could also be % or vw (for example).

Problem

The Diff File header in Merge Requests does not stick to the top of the screen as you scroll.

Cause

CSS calc() cannot have unitless values in the expression.

Fix

Add px to 11.

Notes

As we (rightly should) switch to more native CSS and less Sass, we need to be careful that we understand the differences in those engines so that things like this don't slip through.

One especially tricky part of this is that we're still using Sass to interpolate variables into the stylesheet, which are then ADDED by the native CSS engine. There are a lot of potential vectors for unitless values to slip in.