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.
[Nitpicky me, nitpicky me...] We call it "sign in" in our UI, but we call it "login" in our code. I love when the domain model is consistent the whole way through, so that everyone who talks about the app uses the same terminology (the devs, the business folks, the users, etc). Feel free to change all occurrences of "login" to "signin" if you care. If these feels like bike-shedding, well, maybe it is. In that case, ignore it. It's fine. :bow: :bow: