Migrate to MIT license

Adapted to the MIT license migration of the main "Nord" (1) project.
Detailed information can be found in the main task ticket.

All license SVG badges and the "LICENSE.md" content have been updated.
In addition all source code comment header have also been simplified.

References:

* (1) https://github.com/arcticicestudio/nord

Subtask of nordtheme/nord#55
GH-3

Author: arcticicestudio