Refactor javascript and CSS to use npm, webpack, and remove aiohttp_themes

[storborg]

End goals:

• Use webpack for asset compilation, both js and css
• Don't include vendor components in the git repo
• Build and package static assets when publishing a release to pypi

[storborg]

This is done in bc7c0334d8c7280475e3fac611a1eb710d78af26

Created #29 and #30 for some related tidying tasks.