Fixed the currency_convert engine.

Continuation of PR #1004 (seems abandoned)

results: make engines of results set

Previously, if a same result was returned from the
same engine, the name of the engine would be added
more than once. To eliminate engine name duplication
engine list was changed to set.

[add] OpenAIRE publication engine

and modifies OpenAIRE datasets engine

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