#15 user's 'selected' object is no longer accessible

[amit barak]

scenario:
- user 'selects' (in explore controller) other user's folder she has access permit to
- user closes browser
- other user revokes access permit to the said folder
- user wakes up in the morning and re-opens browser
- explore controller attempts to retrieve the stored 'selected' object, but since access permit is revoked, a nil is returned
- controller does not detect this condition, and sends nil instead of a folder object to the folder rendering template
- template crashes trying to access folder fields or methods on a nil object...

proposed fix
- go over all controller methods that could potentially render folders, and make sure folder is valid before sending it
to template. if folder is not valid, select user's 'home' folder