fix media query edge case (#13546)

fix media query edge case (#13546)

* fix media query edge case

was failing for 768px width before

* code review

* define responsive breakpoints as less variables

* add missing variables.less

* rename variables, drop screen constraints

* fix less import

* Apply suggestions from code review

Co-authored-by: silverwind <me@silverwind.io>
* code review

altough it doesnt matter, LESS lazy evals variables

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>