updated views.py and admin.jinja2

changes in admin.jinja2
- removed dublicate links to jquery and jquery-ui

changes in views.py
- updated signup_get view:
- - - doesnt allow to access this views, if authenticated
- minor changes in all if statements
- changes in some if statements to be used with ternaty operators
- removed encode('latin-1') in some places
- updated add_category_display:
- - - when adding category to courses you need to choose appropriate
category from library categories
- - - showing categories from library only if they are not in courses
categories

- updated edit_category_post
- - - allows to change name of category
- - - if you don't set new name, old one will be used