Implement CDN option

Problem/Motivation

For easy tests and simplytest.me environments in dependent modules (project_wiki) in our case, it would be great to have a "Load library from CDN" option (opt-in) in a general settings page.

If enabled, it should load the library via CDN through libraries alter, like we did in other projects.
The status message should be adjusted accordingly to inform about the cdn option.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes