Migrating away from postgres CI fields.

Caveats:

Assumes fields in LCI fields are all already in lowercase. No efforts are currently made to manipulate these fields. Enforcing lowercase during the migration is doable if required, but the existing implementation of these fields use get_db_prep_save to guarantee only lowercase data is written.
Email fields are migrated to models.EmailField. Case insensitivity is not preserved.
Needs (much) more testing.

Implementation details:

Creates two LowerCase-fields to replace the LCI-fields, LowerCaseCharField and LowerCaseDNSField. The latter inherits from the former. These fields will, like the LCI fields before them, save their data in lowercase by using get_db_prep_save.
Uses a manager to hook into the calls to objects.get, objects.filter, and objects.exclude to ensure they all use lowercase searches into the LowerCase fields. This functionality relies on all fields that are to use lowercase inherit from mreg.fields.LowerCaseCharField.
Uses a mixin for views to overload get_object for relevant detail views.

Noise:

Overloaded get_queryset() usage with prefetching has to manually use lower() on the relevant input due to the manager not applying to querysets. For django-filter, this is solvable by mixing in a filter class that does the lowercasing were applicable, but url_filter does not support this easily.

Fixes #489

unioslo / mreg