Visual tweaks

This should be good for launch. 

I tried to list all the changes I've made but may have missed a few. Also, some of the tweaks might be good to move into the common SASS module once I have more familiarity with the entire system.

# Changelog

### General
* External link icons are less strong
* Download actual bold typeface for Nobile
* Specify lighter weight bold typeface for Ubuntu
* Tweak global font size for better readability
* Override Bootstrap's heading margins for better balance

### Navigation
* Tweak navigation buttons to prevent it from overflowing on medium screen widths
* Menu icons use Font Awesome's fixed width class so that it lines up properly
* More horizontal margin between dropdown arrows and text
* Subheadings on dropdown lists are visually separated from previous links, and are not text-selectable

### Splash
* Increase breathing room
* Add some color
* Fix typo
* Text will not flow underneath star icon on small screen sizes

### Code
* Tooltip on "edit request" icon is placed to the left of the icon to prevent it from going off the edge of the page
* Rewrite code try-it block to prevent API examples from breaking the width of the page
* Tweak styling of code response block and remove button

### index.html
* Fix body padding on mobile screen
* Fix "Data Publishers" row to use proper Bootstrap column alignment
* Prevent data publishing guide link from line breaking
* Trailhead links use Font Awesome's fixed width class
* Trailhead links shouldn't underline the space before the link text
* Enlarge search box width
* Enlarge lede text slightly

### Footer
* Align text and logo consistently
* Tweak line-height
* Fix width issues

### Documentation
* Prevent D3 example from breaking out of the width of the page by horizontally scrolling the chart, rather than the page
* Trailing spaces at the beginning of highlited code blocks are inserting a blank line, so I've removed them where I noticed it

### libraries/
* Remove left side padding
* Use Bootstrap fixed-width icon and remove underlined space on link
* Various other layout fixes