Move images from common to LMS that are only used in LMS

OPEN-328
Remove image that is duplicated in LMS and CMS

OPEN-328
Remove duplicate image files

spinner.gif and spinner-on-grey.gif are the same file.
Explicitly give the LMS a spinner.gif file.

OPEN-328
Move wiki icons to the LMS, where they are used

OPEN-328
Rename pl-faculty.png to placeholder-faculty.png

OPEN-328
Replace 'mit_dome.jpg' and 'simple_graph.png' with a default placeholder image
Merge pull request #6135 from edx/sarina/categorize-common-images

Clean up common/ images - moving, deleting, renaming as necessary