Rename display mode tables

[soininen]

entity_class_display_mode -> display_mode display_mode__entity_class-> entity_class_display_mode

Added a migration script and renamed all associated stuff (queries, mapped items, import/export functions etc.)

Resolves #442

Checklist before merging

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.02%. Comparing base (6130c3b) to head (8c3d3fb). Report is 2 commits behind head on master.

Files	Patch %	Lines
...6ae0f8f_rename_entity_class_display_mode_tables.py	96.77%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #443 +/- ## ========================================== + Coverage 78.96% 79.02% +0.05% ========================================== Files 76 77 +1 Lines 9960 9991 +31 Branches 2127 2130 +3 ========================================== + Hits 7865 7895 +30 - Misses 1746 1747 +1 Partials 349 349 ```

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