Centralize all MooseDocs media into large_media

[aeslaughter]

Reason

Currently, there are various media directories. For an organizational point of view and to avoid a slow bloating of MOOSE, all images and movies should be moved to large_media.

Design

large_media should be organized with a folder for each module: framework, level_set, phase_field

Then each module doc/content folder should contain a media directory that is a symlink to the large_media folder.

Then create pre-check that will prevent images from being checked in, with a nice error message linking to some documentation on using the large_media repo.

Impact

This will make adding images a bit more difficult, but make things much more consistent.