Set the ASSET_URL to be relative to the 'theme/' url, which requires to use
{{ SITEURL }}/{{ ASSET_URL }} in the template and make it works with both
relative and absolute urls.
remove the unit tests for the lesscss generator (TestLessCSSGenerator class) and
replace it with tests for webassets:
- add a sample scss file
- run pelican with example files
- compare the generated css file with a reference one
- look in the html code for the correct link tag