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

• Documentation (also in Toolbox repo) is up-to-date
• Release notes in Toolbox repo have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• 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 ⚠️
...eff478bcb_add_active_by_default_to_entity_class.py	95.45%	1 Missing ⚠️

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Additional details and impacted files

@@             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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