Even before the refactorings, the comment and thread elements were also used
from views outside of diff/. Now they are now completely independent from
gr-diff and descendants, and can move into their own folder and drop the -diff
from their name.
The dependency left from diff/gr-diff-host onto comment[-thread], makes sense
since gr-diff-host is the Gerrit wrapper for gr-diff with gr-comment[-thread].
Change-Id: I5076428da980198989edc605e5edc0e4d66529dd
9b94616a54