Inline code coverage: Make no-coverage lines more prominent than covered lines

Problem to solve

In the merge request diff, when displaying coverage data as red/green lines next to the line numbers, lines without coverage are less prominent than lines with coverage. In most cases where coverage data would be helpful to see next to the merge request diff, it is more important to see which lines don't have coverage.

Proposal

Swap the widths of the inline coverage lines (2px and 4px):

current	proposal

The lines should remain different widths in order to make them distinguishable based on something more than just color. The current colors and line widths were established in #278469 (closed) / !49724 (merged).

Intended users

• Sasha (Software Developer) would like to see more prominent warnings about lines without test coverage when writing and reviewing merge requests