Import only less mixins to avoid reapplying reset

topcrashers and explosive signature reports import base.less to get access to
the contents of mixins.less, which causes reset to get reapplied and overwrite
some of the settings in screen.less

Switch to importing only the necessary mixins class brings these two reports
in line with the visual appearance of the other reports.
Merge pull request #2445 from lonnen/1087287-mixin-imports

Import only less mixins to avoid reapplying reset