Fix image finder

Fixes regression caused by 33b1d27ba94d5f73231d791bbc498b2b15c3c691 in the  can_access_image method
- old method used filesystemfinder, which erroneously searched for images in settings.STATIC_DIRS (/groups/pharmacoresponse/data/images)
- new permissions on /groups/lincs prevented the web client from finding files there
- new method uses os.access method on the settings.STATIC_ROOT directory (docroot/_static)