app: Use current_app.babel_instance instead of babel

That way we can use Babel.init_app() later
Added "Not Available" page
Imported "About" page
Imported "History" page
Imported "Nachruf" page
Added external-link.png images where appropriate
Imported "Contact" page
Imported "Legal Terms" page
Imported "RSS" page
Imported "Contributing to the Database" page
Imported "Legal Terms / Website" page
Imported "Legal Terms / Contribution" page
Imported "Legal Terms / Contributor Terms" page
Imported "Legal Terms / Terms of Use" page
Imported "Legal Terms / Privacy Policy" page
img: Renamed images and added logo without text
Imported "FAQ" page
Imported "Downloads" page
Updated links in "Contributing to the Database" page
i18n: Updated translation template