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
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.
Content field in Arch Wiki search results is of no real use, more often
than not it contains no usable information and includes too many markup
tags which make the text unreadable. It is safe to remove it.
