Replace hardcoded sizes on the Blame page

The following discussion from !111740 (merged) should be addressed:

• @arfedoro started a discussion:

  question (non-blocking):

  Gitlab css grid-size is based 8px so having 5px and 10px padding looks a bit out of general approach.

  Maybe it make sense to use variables with a slightly bigger but more consistent to general approach padding?

  Something like: padding: $gl-spacing-scale-3 $gl-spacing-scale-4;

  This is definitely, if is is agreed that it is an issue, can be addressed in a follow-up and doesn't block approving