Various fixes

See merge request !243

This avoids the need to build the psycopg2 packages and thus requires
PostgreSQL dependencies.

The latest version of the sort method requires a key parameter.

Py3 compatibility

See merge request !242

It's not used anywhere in the code.

Use list comprehension and a helper function so if needed in the
future, only one place will need to be modified.

Use list comprehension and a helper function so if needed in the
future, only one place will need to be modified.

Use list comprehension and a helper function so if needed in the
future, only one place will need to be modified.

The goal is to reduce the number of query created to retrieve the
presentation of the the cache objects associated with the Result
objects.

Use list comprehension and sets.

Replaced map/filters combo with list comprehension and
exploit Django's ORM features like values_list to generate
all these lists.

Replaced map/filters combo with list comprehension and
exploit Django's ORM features like values_list to generate
all these lists.

Replaced filter with list comprehension.

Replaced map/filters combo with list comprehension and
exploit Django's ORM features like values_list to generate
all these lists.