Link to Wall if enabled, and User talk if disabled in wall history message edits

On a wiki with Forum enabled but Message Wall disabled, on Forum thread history, when a username appears as a possessive adjective in an entry, the embedded link is to Message_Wall:[username] rather than User_talk:[username].
https://wikia-inc.atlassian.net/browse/VOLDEV-39
Cache whether wall is enabled
wfMsg -> wfMessage & globals usage

This commit replaces the use of deprecated wfMsg with wfMessage(...)->text() & prefers RequestContext to $this->app->wg->... if possible.
Escape parameters
Remove html-only messages