Removed Devise, added basic authentication

Here is what I did:

I created a migration to get rid of the fields that Devise left behind. I also got rid of the Devise files.
The basic authentication is there. Data sets, sources, and tags can not be modified or destroyed by non-users. It will just redirect you to the login page.

One caveat: If you login with the wrong password, it will give you a 'template missing' error instead of returning you to the login page. I cannot figure out how to display the error in the view using form_tag .

I know we talked about users only being able to delete their data sets, sources, and tags and not others', but I would rather set up Rspec and get some tests up before I tackle those issues.
If you do an `email_field` instead of a `text_field`, then you'll get a fancy HTML5 `type="email"` field. Then you can set `required: true` on it, and it'll add a `required="true"` attribute. 

Why is this the friggin' bee's knees? My favorite reason is because newer browsers will automatically enforce these constraints. If a user doesn't enter something and hits "save", even with no Javascript, the browser will refocus that field and maybe give a message like "yo, you need to fill this in, dawg". And if they enter something that doesn't look like an email address, it'll complain to them about that, too.