mixer: dump database fixes

* Adds new command `inveniomanage database dump` to allow dumping the
  content of the database using the mixer.

* Fixes imports and trailing white spaces on json dump.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
28:1: F401 'six' imported but unused
31:1: F401 'current_app' imported but unused
39: D290 White spaces around docstring should be trimmed.
115: D102 Docstring missing
137: D290 White spaces around docstring should be trimmed.
141: D102 Docstring missing