Added Bootstrap and query. Refactored leaderboard --> Engagement Index

Added the bootstrap gem, and manually added the jquery file in the vendor's folder. Also added a tiny bit of bootstrap onto the engagement index page to test out the bootstrap

@NolanChan @coyote we should never copy / paste external JS files in any rails app. Instead, try to use a gem that's available, or create your own.

e.g. have a look at https://github.com/ets-berkeley-edu/angular-gem