make 600 our default bold font-weight
Issue idea came from: https://gitlab.com/gitlab-org/gitlab-ce/issues/33979#note_33048677
Problem
Currently it is easy for devs to assign the wrong font-weight to text that are bolded in UI mockups. This is because we operate with 2 different values: 600 (semi bold) and 700 (real bold).
Proposal
Can we simplify, and just use 600?
CC/Call to actions
@annabeldunstone @tauriedavis @pedroms (as we were all involved in the change to system fonts ;) )