adminutils: initial release

* Adds Flask-Admin support in Invenio to easily create web administration
  interfaces for SQL Alchemy models.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
installation: secret key creation fix

* Fixes problems with running `inveniomanage config *` without
  CFG_SITE_SECRET_KEY being manually set before. One is able to
  overwrite application config variable SECRET_KEY by passing equally
  named argument to application factory even if the global variable is
  not set.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>