Add sub-categories

[php-coder]

Thoughts:

• it shouldn't be possible to create sub-category with the name of category (and opposite)
• category "Prehistoric animals" should become a sub-category of "Фауна" category (should we update existing mapping?)
• top-level categories should be visible as optgroups and it shouldn't be possible to chose them

TODO:

• prototype of the /series/add page
  • "Animals" category should be removed
  • "Fauna" > "Prehistoric animals" category and sub-category should be created
• /series/add page
  • add support for showing categories/sub-categories
• Migrations:
  • add category.top_category_id field (nullable, foreign key to top_categories.id)
  • "Fauna" category should be created
  • "Prehistoric animals" should be sub-category of the "Fauna"
  • "Sport" category should be added (test data only)
  • "Animals" category should be removed (test data only)
• /category/add page
  • add select "Parent category" (and hint that if it's empty than we'll create a top-level category)

TESTS:

• /category/add, /series/add: re-display of selected category in case of error
• display of selected category when user is opening /series/add/category/$category
• integration test for adding sub-category

Notes:

• we can create separate table for top-level categories (resolution: let's keep it simple and use single table for a while)
  • [pro] we shouldn't modify existing SQL-queries
  • [pro] less chances to break something
  • [con] we have to write separate validator for checking uniqueness
  • [con] more code will be added (migration, service, dao, tests)
• @Category — in one case we need only parent and in others only children. Currently, we always looking for any category
• categories and subcategories vs top categories and categories