Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Implement a scrapper for DuckDuckGo-Lite [1].  The existing DuckDuckGo [2]
engine does not support paging.  DuckDuckgo-Lite is much faster, less verbose
and does have a paging option (reversed engineered from the input form of [1]).

[1] https://lite.duckduckgo.com/lite
[2] https://duckduckgo.com/



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

settings.yml:

* outgoing.networks:
   * can contains network definition
   * propertiers: enable_http, verify, http2, max_connections, max_keepalive_connections,
     keepalive_expiry, local_addresses, support_ipv4, support_ipv6, proxies, max_redirects, retries
   * retries: 0 by default, number of times searx retries to send the HTTP request (using different IP & proxy each time)
   * local_addresses can be "192.168.0.1/24" (it supports IPv6)
   * support_ipv4 & support_ipv6: both True by default
     see https://github.com/searx/searx/pull/1034
* each engine can define a "network" section:
   * either a full network description
   * either reference an existing network

* all HTTP requests of engine use the same HTTP configuration (it was not the case before, see proxy configuration in master)

* searx understand "!ddg !g time" as : send "!g time" to DDG
* !g a DDG bang for Google: DDG return a HTTP redirect to Google

This commit adds a the allows_redirect param not to follow HTTP redirect.

The DDG engine returns a empty result as before without HTTP redirect.

After the main request, send a second to https://duckduckgo.com/t/sl_h

See https://github.com/searx/searx/issues/2259

The language_support variable is set to True by default,
and set to False in only 5 engines.

Except the documentation and the /config URL, this variable is not used.

This commit remove the variable definition in the engines, and
set value according to supported_languages length: False when the length is 0,
True otherwise.

Close #2485

move meta information from comment to the about variable
so the preferences, the documentation can show these information

use
from searx.engines.duckduckgo import _fetch_supported_languages, supported_languages_url  # NOQA
so it is possible to easily remove all unused import using autoflake:
autoflake --in-place --recursive --remove-all-unused-imports searx tests

- remove paging support: a "vqd" parameter is required between each request. This parameter is uniq for each request
- update the URL (no redirect), use the POST method
- language support: works if there is no more than request per minute, otherwise it is ignored !

compile XPath only once
avoid redundant call to urlparse
get_locale(webapp.py): avoid useless call to request.accept_languages.best_match

This reverts commit 4d177039.

Add match_language function in utils to match any user given
language code with a list of engine's supported languages.

Also add language_aliases dict on each engine to translate
standard language codes into the custom codes used by the engine.

Add languages supported by either all default general engines or 10 engines.