VOLDEV-39: Some username links in Forum thread history point to Message Wall on non-Wall wikis

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].

The changes also change some globals usage to RequestContext usage, e.g. `$this->app->wg->Title` to `$this->getContext()->getTitle()`.