adding courses search, courses status change, minor admin template fixes

changes in views.py:
- courses search allows to search by:
- - - category
- - - author (usernames f_name and l_name)
- - - course name
- - - description
- response of the search will be json

- courses status change allows to change status of course between
activated and deactivated

- imported re, it will allow to work with regular expressions (used in
courses search)