Adds FeatureRequest model.

A feature request:
  * must belong to one requester (User type)
  * must have a description with a length within 5..600 chars.
    * Limit inspired by StackOverflow comments constraints.

A user has many feature requests.
User can upvote or downvote a feature request. [Models only]

* Vote provides helper constructor methods for upvote and downvote:
  * Vote.create_upvote(attributes)
  * Vote.create_downvote(attributes)

* Vote.total_for(votable) returns the sum total of upvotes + downvotes

* Vote must belong to a :voter (User type).
* Vote must belong to :votable (polymorphic)
  * Makes it really easy to make any model "votable" in the future.
A user can submit feature requests.

Enables a real-world user to create new feature request.

* Voting not yet available through UI (yet).
* Adjusts a few minor css classes for the nav bar buttons to render correctly.
* Reorganizes Ambience Button in navbar (hides on mobile device)
* Adds Timestamps to FeatureRequest
Updates README todos