Initial recommendation engine

[achrinza]

Recommendation engine will be used in two locations:

1. Homepage hero
   
2. Diary "charts" section footer
   

This issue is to track creating a basic recommendation engine interface for these UI components to retrieve info.

Note that "Log your Sleep" and "Log your Day" in the homepage hero are not by the recommendation engine. But it does mean that the recommendation engine should be able to generate between 1-3 (inclusive) recommendations.