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

## Current Value of `detective.settings.PLAN`
```python
PLANS = [
    {"Free"       : {"max_investigation" :  3, "max_entities"  :  100 }},
    {"Low"        : {"max_investigation" :  5, "max_entities"  :  500 }},
    {"Medium"     : {"max_investigation" : -1, "max_entities"  :  1000}},
    {"High"       : {"max_investigation" : -1, "max_entities"  : -1   }},
    {"Enterprise" : {"max_investigation" : -1, "max_entities"  : -1   }}
]
```

## Results of plan assignations on Staging

```
Creating Profile for Users
smk           ---> Free
another_one   ---> Free
peter         ---> Free
nkb-test      ---> Free
horst         ---> Free
nkb.fr        ---> Free
detective     ---> High
blabla        ---> Free
pbellon       ---> Free
nutzer        ---> Free
test_buddy    ---> Free
toutenrab     ---> Free
florian.gossy ---> Free
test_user     ---> Free
edouard       ---> Free
annelise      ---> Free
paulloz       ---> Free
pirhoo        ---> Free
sm_kraus      ---> Medium
edou4rd       ---> Free
nkb           ---> Low
```
@smkraus wins !