Ability to customize code font size

Based on conversation here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8157#note_20304082

Everyone can have very different preferences on code font size. Should we consider allowing people to customize it?

If so, we could consider either

1. Adding an option in https://gitlab.com/profile/preferences, next to where you can chose Syntax highlighting theme
2. Allowing users to 'zoom in' and adjust on the code viewer itself.