Feature/separate users into tiers

## New Features

- DetectiveUserProfile model added with a `plan` attribute
- Data migration (number 0020) which assigns a plan related to properties defined in `detective.settings.PLAN` (see below)
- Show the current plan for each user in administration
- Dynamic attribute `entities_count` added to `Topic` model
- Show the value of `entities_count` for each topic in administration

## TODO

- [x] Apply migration on staging
- [ ] Apply migration on production