HG-110 Custom smart banner

https://wikia-inc.atlassian.net/browse/HG-110
https://wikia-inc.atlassian.net/browse/HG-215

Related to https://github.com/Wikia/app/pull/5385
We use this convention everywhere. I've seen `on...` in some articles I've read but nothing like that in our code. I'll add comments to tricky functions like this one.