Implement frontend for persona creation and selection

[ChristianSi]

Extend the logged-in widget

• user can create a new persona with an arbitrary name (unique over all users, obviously)
• user can select a different persona from the all personas she created
• the persona selected during the last login sticks, and will be chosed after the next login
• initially, no persona is selected (or even created). this state can be re-entered by deselecting the current persona.
• if no persona is selected, adhocracy sees un-authenticated requests (indistinguishable from "user is not logged in with thentos")
• aim for a trivial, secure UI design. a convenient one will be provided by the design team later, and/or with US 3.

EDIT: typo