Selenium notebook tests

[Conengmo]

Do selenium tests on our example notebooks. The steps are:

• execute the notebook so it's using our up to date code
• convert the notebook to html
• extract iframes from the html
• store each iframe in an html file
• load each html file with selenium
• check if the map loaded
• verify there are no errors in the JS console

After merging this we can see what other tests can be discarded.

In the contributing guide I removed the part about Firefox. We still use it in the code but those tests are marked to allow failure. I want to revisit that code after this.

Reuse the WebDriver instance so the cache is preserved. This means doing less requests for static content.

[amit1rrr]

@Conengmo @stickler-ci Sorry to intrude but I noticed this PR contains Jupyter notebook files that are hard to review on GitHub due to the JSON diffs.

You might want to checkout ReviewNB, code review tool for notebooks. Provides visual diffs & commenting for Jupyter notebooks on GitHub. Completely free for open source repositories.

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