Use pylibzim to create ZIM

[satyamtg]

This fixes #53 and uses pylibzim to create ZIMs. It currently relies on https://github.com/openzim/python_scraperlib/pull/34 and thus has a requirement from that branch itself. Also fixes #24 which was necessary to make pylibzim work.

Openedx instances have many root-relative links and we correctly fix them to be not root relative but just relative if the page that it points is present in the ZIM or else point to an external URL by adding the instance netloc.

The following changes are made in scraper.py related to link rewriting -

• Rewriting is done just after the dependency download is complete
• In order to avoid running link rewriting, before having a list of all offline tabs, the previous annex() method is renamed to get_course_tabs() which only gets the course tabs and the new annex() method actually downloads the content. get_course_tabs() is reused in rewrite_internal_links().
• Internal links are basically of 2 types, one directly pointing to an xblock (with a jump_to in the path (see the xblock_json)), and other one pointing to a tab (for which path is determined by get_course_tabs() as we do not offline all tabs).
• handle_jump_to_path() compares the jump_to type URL and finds the xblock with that URL from the list of xblock_extractor objects, and checks if the xblock is a vertical or course and returns the modified link. As only course and vertical have HTMLs, we look at the descendants for linkable xblocks too here.
• relative_dots() prepares a path of backward jumps, according to the number of parts in the path
• update_root_relative_path() writes ensures that no root relative URLs are left out by putting the instance_url in place of the netloc.
• rewrite_internal_links() is the main manager method. It calls the other functions. In case of jump_to links, if in the first try we do not get a path, we try with the parent as it may be pointing to an xblock with which the vertical xblock is made.

Note this depends on a future release of zimscraperlib

[satyamtg]

Codefactor may complain about complexity (my editor didn't but config is different). In this case we'd moved those methods back

Codefactor doesn't complain so kept it there. Also, the change broke it as the links starting with path_prefix were never fixed, as the absolute links were fixed before. So fixed that.

Also made another module html_processor.py and moved all HTML parsing, dependency downloading and link fixing there. I did this because the scraper.py went too long. Also renamed dl_dependencies to dl_dependencies_and_fix_links and added docstrings for them. Changed the attribute checking for wiki and forum and check value of self.wiki or self.forum now.