Improve extraction (Closes #7918)

Improve extraction (Closes #7918)

remove outer parentheses in if

Conflicts:
	youtube_dl/extractor/imgur.py

checked code with flake8

not returning list in case of single images.

using the fact that id with length 5 are albums and more are single videos.
Also for single videos ie ImgurIE both urls - http://imgur.com/gallery/oWeAMW2 and http://imgur.com/oWeAMW2 are equally fine. Change regex to allow thuis.
For albums urls - http://imgur.com/gallery/Q95ko and http://imgur.com/Q95ko are ok. Change regex to allow this also.

update description in ImgurIE Tests.
Also move single video test 'https://imgur.com/gallery/YcAQlkx' from ImgurAlbumIE to ImgurIE.