Add some basic integration tests to verify that the notebook and server views are usable

We currently have some server-side user tests which verify that the server views return valid html with the expected json/script content, but no real verification that the end result is usable by the browser. Today that resulted in a seemingly-innocuous code mirror change (#1341) breaking our deployment.

We should add some basic selenium tests that verify things are still working end-to-end from the point of view of a Firefox and Chrome client. To start with, I would recommend minimal tests for all the server side views, including the notebook. Even just testing that the basic divs that you'd expect are in place would go a long way towards giving us confidence that (e.g.) a renovate PR didn't break something important.

We could use the treeherder selenium tests as a model for this (the original set just did as above, though it seems like they have since been expanded/extended a fair bit):

https://github.com/mozilla/treeherder/blob/45b2493d06e279a0ed0add0e04bfbb9d71ca6510/.travis.yml#L55
https://github.com/mozilla/treeherder/tree/45b2493d06e279a0ed0add0e04bfbb9d71ca6510/tests/selenium

Start by reviewing the existing server-side user tests, then use the referenced treeherder .travis.yml and tests/selenium examples as a model. Done means adding basic Selenium coverage for the server-side views, including the notebook, and verifying expected elements in both Firefox and Chrome.

Written by the indexing model from the issue text.