Fix bing image thumbnail path

- Forget one "\" in the process!

Instead of a single line with 500000 characters use nicely formatted JSON.
Sort the lists in engine_languages.py so when updating it is possible to
more easily see the differences (search engines do change the order their
languages are listed in)

[add] seznam engine

Update Dockerfile

- Fix for docker image build error "Could not find a version that satisfies the requirement cffi!=1.11.3,>=1.7 (from versions: )"

Minor changes

Add Gentoo wiki search engine

Searching for english is now giving all pages results.
Instead searching for a specific language different from english
is correctly returning only translated pages for the selected language.

Use new LABEL syntax for Dockerfile

Update Docker image to Alpine 3.8

Create an include for __common__/aboutextend.html for imprint purposes.

[WIP] [engine] Duden.de (German dictionary) 

feat(answers): add sha256 and uuid v4 to random answers

Update Dockerfile to Alpine 3.7

[UI] new youtube icon for oscar theme

manage.sh : minor change

Update dependencies

get_doi_resolver return doi resolver url instead of doi resolver name

Change the local name of the Polish language