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 have never used these variables either…

Headings (`h1`–`h6`) are HTML elements to be used in a semantic way. These variables always seemed to tie structural elements to styling in the wrong way. Personally, I always use Bourbon’s `modular-scale` function to set font sizes, which limits the number of possible sizes to those within a scale.