Filter assets by role

* Implemented filtering of rivers and buckets in the content
view using the roles tabs i.e. "All", "Managing" and "Following"

* Added AssetList#following in assets.js for filtering the asset list
based on the 'subscribed' property
Delete assets from content view

* Implemented deleting of assets using the "Delete" button
on the content view in the user's dashboard. A delete action
triggers a confirmation message and a subsequent (success) system
message when the action is confirmed and successfully performed.
Refs #320