Make entity and value metadata use class_name as unique key

[soininen]

class_name is required to uniquely identify entities and parameter values.

Fixes #328 Re #318

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: 2 lines in your changes are missing coverage. Please review.

Comparison is base (3f785a0) 77.05% compared to head (4f56747) 77.06%.

Files	Patch %	Lines
spinedb_api/db_mapping.py	0.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #330 +/- ## =========================================== + Coverage 77.05% 77.06% +0.01% =========================================== Files 74 74 Lines 9200 9201 +1 Branches 1944 1944 =========================================== + Hits 7089 7091 +2 + Misses 1779 1778 -1 Partials 332 332 ```

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