update admin template to support category management

changes in admin.jinja2:
- new tab for category management
- link to add new category
- button to edit category
- button to remove category (doesn't work for now, need new row in db
table)
added add_category view, template to add category, update __init__.py

changes in __init__.py:
- route to add_category view
- route to edit_category view
- route to change_status_category view

changes in views.py:
- added categories_list to response of admin_panel view
- add category view (doesn't work for now, probably it has some issues
with db)