Use the default value for is_active when migrating to active_by_default

[soininen]

This PR adds a compatibility transformation to the migration script that adds the active_by_default column to entity_class table. The script picks up the default value of is_active (or equivalent) parameter and uses that to determine the initial value of active_by_default. The default value is set to None afterwards to prevent the transformation from overriding active_by_default on commit.

This also fixes the analogous compatibility transform for entity alternatives that had a broken entity_alternative table update statement.

Fixes #316, spine-tools/Spine-Toolbox#2535

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[manuelma]

Thank you very much for this @soininen - amazing job!