Fix inclusion of custom CSS in warc2zim2

[benoit74]

Fix #263

Changes:

• Fix and simplify custom CSS injection
  • Pass the HTML code for CSS injection as stuff to be rendered (we need to compute the relative ZIM path based on current page location)
  • directly create a ZIM record instead of faking a WARC record (simplification and less error-prone)
  • store custom CSS at _zim_static/custom.css instead of magic URL warc2zim.kiwix.app/custom.css
• Not related to #263 but a slight enhancement of https://github.com/openzim/warc2zim/pull/260 changes: do not log issues about encoding when only concern is that no content-type was passed in HTTP headers