Enhance Network "get_next" methods to optionally allocate/reserve at the same time

Yet another IPAM-related Network enhancement. Currently calling the `next_network` or `next_address` methods on a Network object will only _show_ you eligible network objects, but will not actually create them.

Let's think about implementing an object flag or alternative endpoint that will allocate or reserve the networks in the same call.

Maybe it could be as simple as implementing support for `POST` to allocate/reserve, where `GET` only shows you what is available?

For example:

See the next two (2) available `/28` networks:

``` http
GET /api/networks/1/next_network/?prefix_length=28&num=2
```

Automatically allocate the next two (2) available `/28` networks:

``` http
POST /api/networks/1/next_network/?prefix_length=28
```