Package collections

**Is your feature request related to a problem? Please describe.**
Users can't create a collection of packages. This would be very useful to create curated lists of packages. For example: Vue SSR (to be embedded on https://github.com/dobromir-hristov/vuecommunity).

**Describe the solution you'd like**
Users can create collections.

Collection properties:
- Name
- Markdown description (optional)
- List of packages

**Describe alternatives you've considered**
-

**Additional context**
-