CSS responsive Grid, remove skel.js dependency

[kittyfishfrommars]

Description

Due to historical reasons this theme relies on the deprecated skel.js library for its Grid system.
A modern pure CSS solution is more versatile and eliminates the flash-of-unstyled-content (FOUC) during rendering.

Acceptance criteria

• CSS supports a responsive grid system, only required parts should be implemented to avoid CSS bloat
• Templates are refactored to use responsive CSS Grid
• skel.js is removed from CSS source files
• skel.js dependency is removed from JS initialization
• skel.js libraries are removed from JS source files

[pmoreno-rodriguez]

Why don't you use Responsive Tools by the same author, referenced in the Skel.js repository?

My Editorial, Future2021, and Telephasic themes use it. If you'd like, I can take a look in the next few days to migrate your Oxygen theme to Responsive Tools.

[kittyfishfrommars]

I prefer to let pure CSS handle breakpoints and grid systems.

The flexbox solution of Bootstrap offers certain advantages in terms of forward-compatibility and maintainability.
It should be possible to exclusively implement the necessary CSS classes to achieve a more performant result.

[pmoreno-rodriguez]

Good. That is a refactoring of the entire css. We could try first with all the scss and mixins functions of Responsive Tools, without the JS, adapted to the Oxygen styles