Changes the base typography of Jenkins

- Normalize the font sizes to a base of 14px
    - Tried to aproximate and consolidate font sizes to 12px, 14px, 16px, and other multiples of 4px
    - Setup wizard keeps the old font sizes
- Changed the default text color to #333
- Used 16px instead on 100% for root font-size. It needs to be in pixels, not percentage. The reason for that is that otherwise the elements using a monospaced font will treat is as an em (size would go down to 11px)
- Adjusted margins for the headings
- Fix font sizes and headings on the plugin management pages