Initialize support for offline engines

Config for instance name and autocomplete

Signed-off-by: mathieu.brunot <mathieu.brunot@monogramm.io>

No inline script for oscar and simple theme

Add image format and source information to display - needs changes to engines to actually display something. 

Displays result.source (website from which the image was taken) and result.img_format (image type and size).

Result is styled with result-format and result-source classes. See PR #1566 for an example of an engine which has the necessary changes.

Strip <span class="highlight">...</span> in the oscar image template.

This PR fixes the result count from bing which was throwing an (hidden) error and add a validation to avoid reading more results than avalaible.

For example :
If there is 100 results from some search and we try to get results from 120 to 130, Bing will send back the results from 0 to 10 and no error. If we compare results count with the first parameter of the request we can avoid this "invalid" results.

Engine fixes

before this commit, sometimes there are no results
use a generic user-agent instead of one with the OS "Windows NT 6.3; WOW64"

Before this commit, the existing settings.yml were always replaced.

at the end of test_webapp.py, the monkey patch of searx.search.Search was not revert which lead to side effects on other tests
close #1663

[fix] fix paging for the oscar theme after PR #1640
Close #1664

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

Characters that were not ASCII were incorrectly decoded.
Add an helper function: searx.utils.ecma_unescape (Python implementation of unescape Javascript function).

closes #1372