changes in edit_category_display and edit_category_post

changes in views.py:
- changes in edit_category_display:
- - - added comments and docstring
- - - this view will return list of available categories from Library
that it can be changed to, plus current category

- changes in edit_category_post:
- - - category editing wount work, see comments inside view, it will be
changed soon;
- - - now it accepts library_id parameter, that will be used to edit
course category to use new category from library

changes in edit_category_template.jinja2
- added select options to choose new category from library categories
Merge branch 'master' of https://github.com/Michaluch/rivneITA003
Merge branch 'master' of https://github.com/Michaluch/rivneITA003