Improvement: Allow custom SCSS code for H5P

[abias]

This is a follow-up to #210 / #207 / #166 and similar to #154 / #215:

In the first implementation of the H5P CSS feature, only custom CSS code is allowed to be written for the H5P content types.

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 H5P CSS designer has all SCSS variables at hand, that the SCSS is properly compiled and it is properly cached and served to the H5P modules.