Move generation of clemsondancer's email address link into Helper

* Ensures the email address is consistent everywhere it's used, gives
  a central location to update it for the entire application.
Create static "Fallback" view that's rendered if no Index Page exists

* Shouldn't happen, but the entire site shouldn't break because of a
  mistake.
  * Instead, we present a helpful error message, asking the visitor to
    email Clemson Dancers if anything goes wrong.
Fix Pages#show to correctly find and render the page based on its URL

* the find method was broken, and the test was incorrectly checking
  that the `@page` instance variable didn't exist. This means that
  rendering a page using the page_url was not working.
* Fixed the controller and the tests, along with testing that the
  `pages/show` template is rendered.
Actually include static fallback view.
Do not allow visitors to register for an Admin Account

* Admins are only creatable by the Superuser, which means you should
  not be able to register for an Admin account from the login form.
Admin section is only accessible to logged-in admins.

* The admin section of the site should be protected: only Admins the
  superuser has created should be able to access the section to manage
  Pages and Galleries.
* A new controller, Admin::Sessions was created so that Devise uses
  the admin layout when rendering the signin page. This is because the
  styling for a page's content may be different than what we want for
  the signin page.