LibreJS compatibility

[fuerbringer]

Make the application GNU LibreJS compatible. Labeling as Backlog since #50 should probably be done first.

Have some links in the meantime (will come in handy when implementing):

• https://www.gnu.org/software/librejs/manual/librejs.html#Setting-Your-JavaScript-Free
• https://www.gnu.org/software/librejs/free-your-javascript.html#step3
• https://www.gnu.org/software/librejs/manual/html_node/Free-Licenses-Detection.html