Close: #1830
Signed-off-by: Nicolas Gelot <nicolas.gelot@e.email>

Locale and search language was always defined with english value.

This patch inits the locale on `pre_request` in order to define the
default value of locale and language preferences.

Plus the `best_match` function provided by flask babel library did not
work as expected. So the function `match_language` provided
by searx is used to detect that the language from Accepted-Language
header can be used in searx project.

Close: https://github.com/asciimoo/searx/issues/844
Close: #35, #42

This bug happens only in python3
because map returns an iterator.

- cgi.escape was removed in python 3.8
- also use html.escape in framalibre.py

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

The new url parameter "timeout_limit" set timeout limit defined in second.
Example "timeout_limit=1.5" means the timeout limit is 1.5 seconds.

In addition, the query can start with <[number] to set the timeout limit.

For number between 0 and 99, the unit is the second :
Example: "<30 searx" means the timeout limit is 3 seconds

For number above 100, the unit is the millisecond:
Example: "<850 searx" means the timeout is 850 milliseconds.

In addition, there is a new optional setting: outgoing.max_request_timeout.
If not set, the user timeout can't go above searx configuration (as before: the max timeout of selected engine for a query).

If the value is set, the user can set a timeout between 0 and max_request_timeout using
<[number] or timeout_limit query parameter.

Related to #1077
Updated version of PR #1413 from @isj-privacore

was "port:ip"

Server Timing specification: https://www.w3.org/TR/server-timing/

In the browser Dev Tools, focus on the main request, there are the responses per engine in the Timing tab.

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.

fix bug: TypeError: set(['bing', 'google']) is not JSON serializable, when curl 'http://127.0.0.1:8888/?q=xxxxxxxx&categories=general&format=json'

Closes #1047

[fix] rss feed : the jinja template was not found when the rss feed returns an valid feed including an error.

The exact order is
* first from SEARX_SETTINGS_PATH,
* if not found then from searx code base,
* if not found then from /etc/searx/settings.yml
* if not found an exception stops searx loading

[enh] validate input and raise an exception inside search.py. The exception message is output in json and rss format.