Finish PRs of @jibe-b: pubmed, oa_doi_rewrite, openaire, arxiv

Closes #1045

Closes #1047

This is the continuation of #1048 created by @jibe-b.

Closes #1048

Closes #1050

-

[fix] Missing favicon images in oscar and simple themes

Same change as with the oscar theme.

The `filename` parameter of the `url_for` function doesn't need a leading `/`, or else the resulting URL features a double-slash `//` that throws off searx 0.12.0 with Apache 2.4.25 on Debian, resulting in missing favicons.

[fix] Language support for Bing Images and Videos

3.0 is still alpha and currently breaks when building styles

Also, fix fetch_languages.py so it can run on python3.

preferences: make supported/not supported labels

Also, Time range support display was missing from RTL locales.
Now it is fixed.

Closes #558

- prevent whitespace-splitting of variable expansions
- prevent interpretation of values as flags/options

(mostly)

closes #869
closes #1055

Switch to code-cov

[add] unit_test for base engine

[fix] voat.co search xpath and url fixed

fix hmac python3 compatibility

[fix] static path fixed in templates and webapp, line endings fixed

nyaa.si fixed

[fix] geektimes.ru xpath fix

[mod]generalfile engine removed

[fix] gigablast api fix

[fix] gitlab search fixed for proper api usage