Style footer area

- Centering footer content.
- Adding social icons plus links to profiles.

LP#15202477
Update social share services

We need only a few services. Cf. wireframe mockup.

LP#15202477
Add original size to download on asset view

We use a temporary URL for original assets to download.

LP#15202477
Merge pull request #25 from NationalMuseumofDenmark/FrontendDesign#15202477

Frontend design#15202477
Merge pull request #26 from NationalMuseumofDenmark/master

Update testing with footer and social/download services