Add dark mode

[LunarN0va]

This PR features:

• Applying the preferred color scheme of the user when first visiting the website.
• A dark mode button at the right side of the navbar. This button uses icons from font-awesome.
• Adding an option to disable dark mode. If disabled, the website will only be run in light mode and the dark mode button will not be shown.
• A new library: font-awesome.
• Added unittests voor HeaderBarViewModelTest: these unittests check if the dark mode button shows and doesn't show.
• Fix for CDNTest cdn locations: this test didn't get included when running all the tests, this is fixed now.

Dark mode applied

Light mode applied

Some issues:

• I don't like how the sun icon looks. I'm thinking I just make it a moon.
• PlantUML diagrams don't convert to dark mode. I've been researching what a possible fix for changing the background colours of containers/componens and I think I'm going to look into the source files of the Structurizr DSL to better understand why the background colours can't change.

[jp7677]

May be this PR is a good reason to switch from the c4 to the structirizr exporter since that one plays a bit better with background styling.

[jenspav]

Superseded by #606