Resolve "provide FileSystemStorage independent from default"

Closes #72

See merge request !113

Resolve "ReferenceError: disable is not defined"

Closes #70

See merge request !112

feat(comments): replace notifications/comments system with threaded trees using mptt. Tree level is limited (in views and templates, not generally) to a maximum depth of 1

feat(comment_threads): change Notification and Comment models into subclasses of MPTTModel to allow threading

feat(forms): add template tags to get the parent in comment tree and to dynamically prepopulate a django form instance with template context data

as a measure of security, files containing data that may comprise sensitive information are stored
separatly from usual media files

closes #72

see #70

closes #70

feat(notifications): add combined query for notifications with target user and datasets administered by the user

feat(perms): add User method to query for all datasets a user has admin rights to to use for notification stream

feat(perms): add model function and template tag to check if a user has view perm to restrict access dataset