Hide enabling of site notices behind flag

Follow up to: I95343be08b4fac054f8b31fcdc62dd1340a30bbc
Note this doesn't take into account existing site notices (these
may be cached - left up to project to use MediaWiki:Mobile.css if
desired)

Bug: 65395
Change-Id: I2aec3a8010a438e647612a366e35ef8db39551f7
Merge "Hide enabling of site notices behind flag"