doi_resolvers / default_doi_resolver were missing in the settings_robots.yml file, so the test server was not able to start (crash). Since the output wasn't displayed, it was not obvious why the Selenium couldn't connect to searx.
Preferences class was introduced in order to handle user preferences. Right now
it parses cookies and the form in preferences. Also it can retrieve settings
based on the name of the setting.
ATTENTION
Please note that engine preferences are handled differently from now on. So it
introduces incompatible changes. Every user who has saved preferences should reset and
save his/her settings again.
This change was needed, because everytime a default disabled engine was
added saved user preferences would broke. Now engine setting tracking is
fixed.
