Administer Organizations and their memberships

Add an organization administrative interface with
links to and from the existing membership admin screens.
Offer to link existing account to new organization

Instead of refusing to create a new organization when the email is already in
use, add an additional "relink" checkbox control.  If the checkbox is selected,
add an additional membership for the account and set it as the default.
Add demo flag on Organization management screen
Correct typographical error (duplicate for)
Merge pull request #5 from documentcloud/master

To DC Master