Initialize homepage, with menu

[benlk]

Changes

This pull request makes the following changes:

• adds homepage layout and rough template
• adds homepage LESS and CSS
• adds blank less/variables.less so that the homepage template can reference that
• registers "Homepage Quick Links" menu location for output on homepage top, since that's shown in the wireframes
• does not match any designs, yet, because the designs are not yet approved

Why

For #40.

Testing/Questions

Features that this PR affects:

• Homepage choice in Appearance > Theme Options > Layout
• New menu option in Appearance > Menus > Locations

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Is there any other boilerplate code that needs to be added before we start on design work?

Steps to test this PR:

0. In Settings > Reading, set "Your Homepage Displays" to "Latest Posts".
1. Activate the homepage layout
2. Create a new menu to fit the "Homepage Quick Links" location and populate it
3. Add some widgets to the "Homepage Bottom" widget area
4. View the homepage: are the widgets present? Are the links in the menu present?

[joshdarby]

@benlk This looks good, but it might be worth noting that we need to set the homepage to your latest posts in settings -> reading since their prod site now has the homepage set to a static page.

[benlk]

I'll add that to the go-live documentation as well.