Migrate project to nested structure

[AlexZhukovich]

Currently, the project supports only 1 level of nesting:

• Category 1
  • Article 1
• Category 2
  • Article 1
  • Article 2
  • Article 3

Migrate the project with the structure of infinite nesting:

• Category 1
  • Article 1
• Category 2
  • Subcategory 2-1
    • Article 1
    • Article 2
    • Article 3

[AlexZhukovich]

Merged in PR #35