Vid 2089 Use JSSnippets to load galleries 

**Edit: Hold off on merging this PR for now. I'm still not positive this is the approach we're going to take.** 

This is for tickets like https://wikia-inc.atlassian.net/browse/VID-2089 and https://wikia-inc.atlassian.net/browse/VID-2105, which are for new galleries not loading in places like message wall and article comments, where the HTML is inserted after page load. 

This solution uses JSSnippets to load galleries, and includes an update to JSSnippets that will make it will fire whenever any callback function is pushed to ```window.JSSnippetsStack```. Before, JSSnippets would only work on page load.  @macbre, could you please review this change?

There is another aspect to this ticket that is a fix for message wall. The problem was that the parser instance wasn't getting passed properly to the new media galleries. It is being passed now. @armonr did that part.