Adding sections

[danbalarin]

Fixes #103.

TODO:

• [x] Creating new sections
• [x] Editing section name and order
• [x] Deleting sections
• [x] Creating subsections (materials and excercises)
• [x] Editing subsections (name, content and order)
• [x] Deleting subsections

[tenhobi]

Please, format the code so they are only spaces (e.g. use soft tabs). :-)

[danbalarin]

I'll format it when everything is done.