Issue #3589119: Move combination field warning into .install config

Danil Veselovrequested to merge
issue/unique_field-3589119:3589119-settings-for-combination into 8.x-2.x

Includes the "Move DEFAULT_WARNING_MESSAGE into .install config" commit from the issue #3589117. MR: !58

Remove the combination field warning text from the .module and move it into the .install config.
Add update hook to set the warning text for existing installations.
Provide placeholders for the comma separated list of fields that have to be unique.
Provide placeholders for the URL and label of the entities that would be duplicated.

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