toaster: Move Custom image recipe rest api to api file

toaster: Move Custom image recipe rest api to api file

We now have a dedicated file for the rest API so move and rework for
class based views. Also clean up all flake8 identified warnings.

Remove unused imports from toastergui views.

The original work for this API was done by Elliot Smith, Ed Bartosh,
Michael Wood and Dave Lerner

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>