Fix canRender and add greppable messages for dynamic keys
Fix canRender and add greppable messages for dynamic keys

This fixes issues raised in https://gerrit.wikimedia.org/r/#/c/416207/4/includes/EchoCoreThanksPresentationModel.php

Bug: T192711
Change-Id: I5e654f5ae9480aa048326c067129fafd430f0515

wikimedia/mediawiki-extensions-Thanks,wikimedia/mediawiki-extensions-Thanks,wikimedia/mediawiki-extensions-Thanks