Template the save code now rejection reason to ease operator workflow

As an incremental step after proposing a rejection reason (#3256 (closed)), template it.

This should be configurable per configuration file so we could easily deploy it later on when adding new rejection reasons.

A first templated list could be (improve the wording at will):

• The origin visit type is not supported
• The origin provided is not an origin with the correct type
• The origin provided is not a repository link
• ...

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Migrated from T3257 (view on Phabricator)