VE-635: Keyboard shortcut for source editor

https://wikia-inc.atlassian.net/browse/VE-635
The WikiaCommandHelpDialog was based on MWCommandHelpDialog which does the same thing with using a MW message for the accesskey. Their usage is a bit different because there's really a button onscreen that has the accesskey parameter set. I don't think users will be modifying this (and it's not in the requirements), so I'm going to switch this to be hardcoded like the rest of the triggers.