Maintain styles and JavaScript as external files

[bittner]

Maintaining CSS and JavaScript as external files should improve maintainability.

• It allows us to effectively run linters over CSS and JavaScript code
• We can edit those files with proper syntax highlighting
• Maintaining minified versions of their code can be done without compromises
• The Python module size (lines of code) is significantly reduced

Fixes #1

[bittner]

I'd like to have acceptance tests running before we merge this change (see #9). This way we should be able to prove that no existing feature breaks.

[bittner]

The newly introduced tests (#9 and #19) were useful to make sure that the refactoring could happen without breaking existing functionality.

All tests now pass again, only checks fail on the behave_html_formatter.html module. They're most likely straightforward to fix, but I'd prefer @schidlo to address them as you know the code better to understand their consequences.

Hence, merging. :rocket: