Update UI for forms and buttons

UI updates to the "Manage Jenkins" and job configuration pages, per discussion on [the jenkins-dev mailing list][list].

 [list]: https://groups.google.com/forum/#!topic/jenkinsci-dev/zDaX4yiWLLw

### Text inputs have larger size, more padding, and rounded corners.

Before (Jenkins version running on Cloudbees):

<img src="https://api.monosnap.com/image/download?id=G0rhktDm0iXVcFK2R8pbETCX0botbY" />

After:

<img src="https://api.monosnap.com/image/download?id=Z4YgRZvy1qyMPcunys0zq0AnENPFLs" />

### Headings on forms are bigger

Now form sections are clearly delineated with spacing, and the size of the heading font - horizontal lines are given less weight.

Before:

<img src="https://api.monosnap.com/image/download?id=PQdWT9f3ptvo12nAsAIMbPASApKEzM" />

After:

<img src="https://api.monosnap.com/image/download?id=UTCk7RTjT3cUvtGXAKTBp3R6vLB6dZ" />

### Updated buttons

Buttons now come in three different colors (from the Jenkins color palette) and have different states for
hovering, clicked, disabled. Dangerous actions are colored red. Buttons have
more padding and use Helvetica instead of Lucida Grande.

This is easier to see the effects if you check out the branch, but here is a static screenshot of the button states. Add the "primary" class to get blue, "danger" class to get red buttons.

<img src="https://api.monosnap.com/image/download?id=FYkxv5KzmbtNgguCNCds2JT0eJdjIj" />

Before:

<img src="https://api.monosnap.com/image/download?id=zYIJaeC5RD1NXQumPlpN8YdGVHct4Z" />

After:

<img src="https://api.monosnap.com/image/download?id=gr43K5pNaQwIFWElM46Dik6G538Oks" />

### Dropdown menu updates

The dropdown menus are slightly larger and have the mechanical-looking yui-menu-shadow replaced with the blur now available with CSS box-shadow.

Before:

<img src="https://api.monosnap.com/image/download?id=N6jM5QMkmOITIM5bQ9GDDcMs34u0qT" />

After:

<img src="https://api.monosnap.com/image/download?id=E5DGvOwNualwt5GeJSf05Rs81nOmF5" />