Message explanations in Engine #1446
Description
Design and implement an Engine API for getting message explanations, and add the explanations from zonemaster/zonemaster#1377.
The Engine API should satisfy the needs of zonemaster/zonemaster#1378. Also, the implementation should consider the needs of developers and translators. E.g., correlating messages and explanations when navigating the source code, and correlating messages and explanations when editing PO files.