Improvement: Allow custom SCSS code for Moodle mobile app

[abias]

This is a follow-up to #205 / #195 and similar to #154:

In the first implementation of the mobile CSS feature, only custom CSS code is allowed to be written for the Moodle mobile app.

It would be worthwhile to extend this in a way that also SCSS code can be used.
The challenge is to build this in a way that the Moodle mobile app CSS designer has all SCSS variables at hand, that the SCSS is properly compiled and it is properly cached and shipped to the Moodle mobile app.