Cache GitHub Projects to avoid rate limit

[awestmoreland]

We're hitting GitHub's rate-limiting when trying to populate the list of projects, which results in no output for that section of the home page.

We should cache the projects list locally, probably no more than once per day.

From GitHub's documentation:

Unauthenticated requests are limited to 60 requests per hour. 
Unauthenticated requests to the Search API are limited to 10 requests per minute.

Unauthenticated requests are associated with your IP address, and not the user making requests. 

Background on this issue:

@jacebrowning reports this console error when testing locally:
GET https://api.github.com/orgs/friendlycode/repos 403 (Forbidden)

{
  "message": "API rate limit exceeded for 152.160.140.86. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)",
  "documentation_url": "https://developer.github.com/v3/#rate-limiting"
}