Issue #3589117: Move DEFAULT_WARNING_MESSAGE into .install config

Danil Veselovrequested to merge
issue/unique_field-3589117:3589117-fill-the-message into 8.x-2.x

Remove the DEFAULT_WARNING_MESSAGE constant and move the text into the .install config.
Add update hook to set the text for existing installations.
Remove the fallback to constant.

Edited by Danil Veselov

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