User: System block reasons shouldn't expand templates

User: System block reasons shouldn't expand templates

The block reasons for "system" blocks shouldn't expand wikitext
templates immediately. That should be left for the code parsing the
block reason for display.

This should only affect how these blocks are reported to API clients, as
when the block is displayed in the web UI it's passed through the parser
anyway. The main drawback, as far as the default messages go, is that
MediaWiki:sorbsreason won't have {{SITENAME}} expanded in
the API response anymore.

Bug: T191939
Change-Id: Ib2024721ea0e26358b9b50efdac16316d6d0f0b6