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.
This will work, but it leaves a key in the user's session for `user_id`, which is probably not what we want.

It looks like we can use `reset_session` to completely purge the user's session, which seems like perhaps the best option. Another option would be to just delete the `user_id` key from the session entirely with `session.delete(:user_id)`.