Graphical an functional modifications
- added a header image
- removed the brand
- changed the colors of the main navbar
- rewritten the form for cource creation functionality, in order to have a
  dedicated page for cource creation/modification
Rewritten the functionality for courses modification
- rewritten the form for courses modification
- rewritten the table that displays the list of courses
- changed the courses and lessons tables structure
- changed the way the lessons are displayed
- improved error messages handling for courses creation/modification/deletion