Refactor css for form elements

I'm going to do this progressively so we don't have a merge nightmare. Basically we should have good defaults on the element level instead of having to use custom classes everywhere. i.e. Buttons should look like stellar buttons by default without having to use a class every single time we have a button. Or having to put ".stellar-form" on every form we create, if that makes sense. 

1. Style and use defaults
2. Override bootstrap classes if necessary
3. Only define custom classes for custom scenarios

This is to ensure good inheritance and increase efficiency. Will welcome any feedback! :)