Migrate settings rewrite

[martastain]

PR Checklist

• [x] This comment contains a description of changes
• [x] Referenced issue is linked

Description of changes

migrate_settings function in BaseServerAddon.convert_settings_overrides is replaced by migrate_settings_overrides.

Technical details

The new function does not return BaseSettingsModel which needs to be postprocessed, rather it returns a dict, so input and output have the same format and it makes it possible to know which values actually have overrides.

The new function drops the original custom_conversions functionality as it was overengineered and not used.

Additionally, modify_settings endpoints now accept __pinned_fields__ list, that enables explicit pinning of default values via settings editor (without pin_override function).