[change] API: use mixins from openwisp-users

[nemesifier]

We should change the code of this module to use the new API classes and mixins added to openwisp-users, which were based on the code from this module so should be backward compatible, however, I found out a few issues:

• some classes in openwisp-users were not taking into account the organization_lookup field, hence queries were failing to find the right organization field, I solved this in https://github.com/openwisp/openwisp-users/tree/improve-api-mixins, but tests are missing, we need at least one test in openwisp-users. Issue: [bug] API mixins don't take into account organization_lookup  openwisp-users#255.
• a lot of assertNumQueries tests are failing! It looks like the number of queries increased between 2 and 3 queries for each endpoint, we should understand what the additional queries are doing, if they're really needed or not, if the extra queries are generated by mistake or not and what we can do to fix it. Issue: [bug] API mixins and classes seem to generate unneeded queries. openwisp-users#256

I started this work on this branch: https://github.com/openwisp/openwisp-firmware-upgrader/tree/openwisp-users-api-mixins, it needs this other branch from openwisp-users to work: https://github.com/openwisp/openwisp-users/tree/improve-api-mixins, I had to give up because it's taking longer than expected and I hope @ManishShah120 you can help me to finish this.

PS: I think we should also change the definitions of the API views to use tuples instead of lists.

[ManishShah120]

a lot of assertNumQueries tests are failing! It looks like the number of queries increased between 2 and 3 queries for each endpoint, we should understand what the additional queries are doing, if they're really needed or not, if the extra queries are generated by mistake or not and what we can do to fix it. Issue: openwisp/openwisp-users#256

@nemesisdesign Total of 38 assertNumQueries are failing, due to the addition of these permission classes DjangoModelPermissions and IsOrganizationManager. An also due to the modification of the get_queryset in two tests number of queries decreased.

Here is a Summary of it:-

The test in which it increases by 2 in those cases It is due to the addition of DjangoModelPermissions permission class from openwisp-users module (becuase we have also added a permission check for the GET requests as well but the tests were written without those checks with the default rest-framework permission class) and those extra two queries are of this sort:-

2. SELECT "django_content_type"."app_label", "auth_permission"."codename" FROM "auth_permission" INNER JOIN "openwisp_users_user_user_permissions" ON ("auth_permission"."id" = "openwisp_users_user_user_permissions"."permission_id") INNER JOIN "django_content_type" ON ("auth_permission"."content_type_id" = "django_content_type"."id") WHERE "openwisp_users_user_user_permissions"."user_id" = '7798476dfa1e4dbcb65674b23c40c096'
3. SELECT "django_content_type"."app_label", "auth_permission"."codename" FROM "auth_permission" INNER JOIN "auth_group_permissions" ON ("auth_permission"."id" = "auth_group_permissions"."permission_id") INNER JOIN "auth_group" ON ("auth_group_permissions"."group_id" = "auth_group"."id") INNER JOIN "openwisp_users_user_groups" ON ("auth_group"."id" = "openwisp_users_user_groups"."group_id") INNER JOIN "django_content_type" ON ("auth_permission"."content_type_id" = "django_content_type"."id") WHERE "openwisp_users_user_groups"."user_id" = '7798476dfa1e4dbcb65674b23c40c096'

The test in which it increases by 1 in those cases It is due to the addition of IsOrganizationManager permission class from openwisp-users module and those extra two queries are of this sort:-

5. SELECT "openwisp_users_organization"."name", "openwisp_users_organization"."is_active", "openwisp_users_organization"."created", "openwisp_users_organization"."modified", "openwisp_users_organization"."slug", "openwisp_users_organization"."id", "openwisp_users_organization"."description", "openwisp_users_organization"."email", "openwisp_users_organization"."url" FROM "openwisp_users_organization" WHERE "openwisp_users_organization"."id" = 'c11e66b739cf477c9786bcc099b954ae' LIMIT 21

and in cases in which there is increase of 3 are due to the combined queries from both of the classes.