[DRAFT/DEMO] lxml free citation2latex

Despite the fact that parsing an html structure using regex is a bad practice, here the lxml approach (#4090) is replaced with a regex. The main idea is, that there is actually no real html structure to parse but rather some html tags with a data-cite attribute.

I'm still not convinced that lxml is necessary. The lxml approach requires the lxml package, is much slower and introduces some bugs which are difficult to cope with.

This fixes #4251 and #4283