Add active_by_default flag to entity_class table

[soininen]

Added active_by_default flag to entity_class table which specifies the default value for entity alternatives of the entities of the class. By default, zero-dimensional entities have the flag set to False while for multidimensional entities, it is set to True.

Since this change alters the entity_class table, we now have a new migration script as well.

Queries, filters etc. were updated, too.

Implements #316

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

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (44b6883) 77.07% compared to head (ab1b8e2) 77.14%. Report is 1 commits behind head on 0.8-dev.

Files	Patch %	Lines
spinedb_api/mapped_items.py	66.66%	1 Missing and 1 partial :warning:
...eff478bcb_add_active_by_default_to_entity_class.py	95.45%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #336 +/- ## =========================================== + Coverage 77.07% 77.14% +0.06% =========================================== Files 74 75 +1 Lines 9205 9239 +34 Branches 1944 1949 +5 =========================================== + Hits 7095 7127 +32 - Misses 1778 1780 +2 Partials 332 332 ```

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