Support category hierarchies

Now that we have more specific categories for apps in fdroiddata, we also have a lot more categories. While it becomes easier to find relevant apps within categories, it now becomes harder to find the right category.

Therefore, I am suggesting to introduce parent/child relationships for categories, so we can present them in hierarchies for hopefully easier visual parsing:

Multimedia
┠ Offline players
┠ Online players
┠ Downloaders
┠ Podcast
┠ Radio

In order to represent this in a backwards-compatible fashion in our data structured, I propose a new optional parent field for categories:

Multimedia:
  name: Multimedia
Offline Music & Video Player:
  name: Offline players
  parent: Multimedia
Online Music & Video Player:
  name: Online players
  parent: Multimedia

This would translate to the index-v2 as follows"

        "categories": {
           "Multimedia": {
                "name": {
                    "en-US": "Multimedia",
                }
            },
           "Offline Music & Video Player": {
                "name": {
                    "en-US": "Offline players",
                },
                "parent": "Multimedia"
            },
           "Online Music & Video Player": {
                "name": {
                    "en-US": "Online players",
                },
                "parent": "Multimedia"
            }
        }

Our code here would make to be sure that the value of parent refers to another category with exactly that ID. If such a category isn't defined, an error should be thrown.