Remove a bunch of redundancy in variables and header text styles

These changes are to  simplify bitters and are opinionated based on building web applications and not web sites.

* Remove header styles because they always get changed. The initial styles just add overhead to be changed. 
* Remove `blockquote` and `cite` styles. I've hardly ever used them and changed them when I have.
* Create `$base-spacing-small` for all the places where we were using `$base-spacing / 2`
* Remove unnecessary duplication in form variables.
* Remove unnecessary duplication in link and button variables.
* Remove `$base-background-color` as it rarely is used more than once.

I'd like to keep `$base-link-color` around, and wouldn't mind if `$base-accent-color` defaulted to `$base-link-color`. 

I think they're two different things.