HG-438 Main page title in Mercury's WikiVariables

https://wikia-inc.atlassian.net/browse/HG-438
Yeah, I've doubled check it and it seems safe. As we discussed it with @rogatty there are: `getText()` and `getDBKey()` methods in `Title` class. The first one gives us the text with spaces and the second one the same text but with underscores instead of spaces according to PHPDocs. In terms of API `getPartialURL()` seems like a right choice but Igor tried it AKAIK and it had spaces instead of underscores. We could always use `str_replace()` but I'm not a big fun of this solution even though we've got 288 usages (58 in wikia directory) of it with parameters: `'_', ' '`...

Let's ask even more experienced with MW people: @Grunny, @macbre do you think using `getDBKey()` fine here?