Modify navbar-toggle to scale better

The navbar-toggle button had a hard-coded size, making it difficult to adjust the size of the navbar. I used some variables to reverse-engineer the current defaults that also look decent with different navbar sizes. I tested by adjusting `@navbar-height` to 30px and `@grid-gutter-width` to 18px (same proportions as the default 50/30). It doesn't rely directly on `@grid-gutter-width`, so changing navbar-padding-horizontal directly should work as well (and look better if the goal is to make the navbar larger).

The bars on the button also didn't respect preferences for border radius.

I compiled the code using grunt to get a new css mapping, but the css compiles down to the same results as before for bootstrap.css and bootstrap.min.css.