New 404, 500 and Heroku Error pages

[petergault]

The pages aren't designed and don't look great.

Some key things to include:

• State in one sentence what Quill is - some people find Quill through a 404, so let's use it as a chance to present Quill.

• Students sometimes get this trying to go to an activity - use this as a chance to direct them back to their profile.

• Key actions are view homepage and return to my profile. This can be hardcoded as quill.org/profile for students.

• Fun illustration.