Make it possible to query top delegates by the number of delegators

Currently, the only way to query top delegates is to get the top `n` delegates by vote weight. Like this:
```
api/delegates/top?space=gnosis.eth&limit=100
```

We must add the option of getting the top `n` delegates based on the number of delegations. The list should be returned in a sorted form.

Start by tracing the api/delegates/top endpoint shown in the issue and identify how the current vote-weight ordering and limit are applied. Add a way to select ordering by the number of delegations while preserving the space and limit parameters. Done means the endpoint returns the requested top delegates sorted by delegator count.

Written by the indexing model from the issue text.