[TASK] Migrate backend styles from LESS to SASS

In preparation for upcomming switch from bootstrap to SASS the backend
style sources have been migrated to SASS. The autoprefixer configuration
was corrected to match CMS8 LTS requirements and minification of CSS
files is now enabled. Automatic style checks are now available through
grunt. The tasks `grunt format` and `grunt lint` will now check sources
files for consistency.

Resolves: #80453
Releases: master
Change-Id: I9ba23aea78014a1f0cce229db533a81d5ffcd02f
Reviewed-on: https://review.typo3.org/52151


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Marco Christian Krenn <krenn@webconsulting.at>
Tested-by: Marco Christian Krenn <krenn@webconsulting.at>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>