Sync upstream v0.17.0

See merge request !60

[enh] add @piplongrun @jjasonkal @KazamaSion @resynth1943 @ahangarha @gordon-quad @999eagle to authors

this commit is picked from #1985

this commit is picked from #1985

Sync to upstream

See merge request !59

* Made first attempt at the bangs redirects plugin.

* It redirects. But in a messy way via javascript.

* First version with custom plugin

* Added a help page and a operator to see all the bangs available.

* Changed to .format because of support

* Changed to .format because of support

* Removed : in params

* Fixed path to json file and changed bang operator

* Changed bang operator back to &

* Made first attempt at the bangs redirects plugin.

* It redirects. But in a messy way via javascript.

* First version with custom plugin

* Added a help page and a operator to see all the bangs available.

* Changed to .format because of support

* Changed to .format because of support

* Removed : in params

* Fixed path to json file and changed bang operator

* Changed bang operator back to &

* Refactored getting search query. Also changed bang operator to ! and is now working.

* Removed prints

* Removed temporary bangs_redirect.js file. Updated plugin documentation

* Added unit test for the bangs plugin

* Fixed a unit test and added 2 more for bangs plugin

* Changed back to default settings.yml

* Added myself to AUTHORS.rst

* Refacored working of custom plugin.

* Refactored _get_bangs_data from list to dict to improve search speed.

* Decoupled bangs plugin from webserver with redirect_url

* Refactored bangs unit tests

* Fixed unit test bangs. Removed dubbel parsing in bangs.py

* Removed a dumb print statement

* Refactored bangs plugin to core engine.

* Removed bangs plugin.

* Refactored external bangs unit tests from plugin to core.

* Removed custom_results/bangs documentation from plugins.rst

* Added newline in settings.yml so the PR stays clean.

* Changed searx/plugins/__init__.py back to the old file

* Removed newline search.py

* Refactored get_external_bang_operator from utils to external_bang.py

* Removed unnecessary import form test_plugins.py

* Removed _parseExternalBang and _isExternalBang from query.py

* Removed get_external_bang_operator since it was not necessary

* Simplified external_bang.py

* Simplified external_bang.py

* Moved external_bangs unit tests to test_webapp.py. Fixed return in search with external_bang

* Refactored query parsing to unicode to support python2

* Refactored query parsing to unicode to support python2

* Refactored bangs plugin to core engine.

* Refactored search parameter to search_query in external_bang.py

[fix] oscar theme, remove inline style attributes (CSP compliants) 

Revise of the gigablast engine

Fix YaCy text results returned as images

add correction support for duckduckgo

[blog post] Developing in Linux containers

update und polish README

Make Oscar theme scalable

Use gender neutral pronouns in docs and blog

Closes #2031

[fix] about page- 'How to set as the default search engine?'