rename .retina-image => .img-retina
Update forms.less

All variables should be set in variables.less?
rm .control-group from docs since it's no longer in the LESS
Merge pull request #8455 from cvrebert/control-group-dead

rm .control-group from docs since it's no longer in the LESS
Merge pull request #8454 from bassjobsen/patch-2

Update forms.less
Merge pull request #8453 from cvrebert/retina-rename

rename .retina-image => .img-retina
Merge remote-tracking branch 'upstream/3.0.0-wip' into 3.0.0-wip