migration helpers for local defaults

[devkral]

Problem:

Sometimes you want to add new fields (note: not column) with a default (edgy) to the database.
This is until this PR quite cumbersome.

The most important change by this PR is adding a help command/API which temporarily make the affected columns nullable and tries to use the default or an additional given default.

Changes:

• null-field or nf parameter for makemigrations/revision.
• The default migration templates allow now to use complex defaults for migrations.
• Fields must use get_columns_nullable instead of ColumnDefinitionModel null. for determining if the columns should be nullable.
• ForeignKeys aren't required to be saved models when passed.
• Cli command revision takes now also the arg argument.
• Revisioning works now with relative revisions with - (e.g. -2).
• Downgrades are now possible with unique_together. Build a constraint name from the fields.
• streamline contentype fields with CURRENT_FIELD_CONTEXT contextvar
• update docs
• fix migration issues with FileFields