Small changes

[Athelena]

• [x] add link to CityJSON and the ninja repo (prominently at the bottom)
• [ ] explain the name ninja under help
• [ ] blue colour for settings and help should either match logo or clash less
• [x] Upload and Browse are not both necessary on the landing page
• [x] X for deselecting an object would be nice

[liberostelios]

Last point was fixed with 3b131ab.

[liberostelios]

First point was fixed with 3aab56d.

[Athelena]

For the first point: the links now only appear after you load a file, but they should already be there at the first page I think.

[Athelena]

Fourth point fixed with 9e4bbbb