Petition detail view and CAS login

[garoller]

Summary of major changes:

• Remove custom User model. Using Django's built-in User model.
• Login using CAS (django-ng-cas) and WTG's identity service.
• Add petition detail view to see author, description, signatures, expiration date.
• Add sign functionality, if a user is logged in and has not signed that petition already.

Minor changes:

• Run travis with the python 3.6 xenial distribution. Travis builds started failing, and this change fixed that.
• Update lock file and instructions in the README for installing dependencies.
• Style changes to the navbar.

Closes #2 and #31.

This is a preliminary design for the detail view, so feedback on the style and how it can be improved would be great.

[jlyon1]

Overall, looks good 💯. A few minor changes around the relationship between petitions and signatures, as well as thoughts on authentication.

On the detail view: Looks good, however it is a little small on mobile, due to the display not collapsing or resizing text. This does appear to be an issue on the site as a whole. Example on iphone X:

Other points that should be fixed at some point but may be outside the scope of this PR:

• The ID can be set and can be negative, negative IDs cause an error on the site
• There is a field labeled '300' on the admin interface which actually represents the number of signatures
• Admins can assign signature to petitions, this should not be allowed.

[jlyon1]

A debug mode would be helpful here so that we can test with different users or bypass cas entirely

[garoller]

I searched around, and there doesn't seem like an easy way to do this. For now, I added that if debug mode is true, then the user is set to a superuser, which at least avoids accessing the database to set permissions.

I think what you suggested is a good idea, but I think we can create an issue for this, as opposed to including it in this PR.