Make /login give a proper JSON response again

Dependencies

• [2] 4AKMU4ZB Clean up user editing
• [3] WDKFN4B2 Make sign in a modal dialog box rather than a separate page
• [4] XJRJ4J7M Add user registration
• [5] LZVO64YG Merge in the first bits of the API work

Change contents

• replacement in src/lib/Hydra/Controller/User.pm at line 33
  [3.380]→[2.52:86](∅→∅)
  − $self->status_no_content($c);
  [3.380]

  [3.24558]
  + currentUser_GET($self, $c);