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 don't use uppercase letters for id's or classes. e.g. here it should be something like `id="engagement_index"`. Also, you probably don't need an `id` here, since we should only style things with `class`