Leave tip design - closes #65

This PR should change the app to resemble the designs in #65. Specifically:

The `textarea` on the leaveReview view should be visually merged with the label above it. This should **not** affect any other text fields in the application.

Also the navbar view now has two more options for configuration:
1. A 'Cancel' button on the left
2. A 'Post' button on the right

This PR cannot be fully approved/merged/tested until codeforamerica/bestnest-api#67 is resolved and the HTTP transaction goes through. Currently it is returning a 400 due to an API problem