Move to numeric need IDs

This pull request makes a change to the Need API to use sequential integers for need IDs, phasing out using BSON IDs. 

The new IDs start at 100001, to avoid conflicting with existing Need-o-tron need IDs. This number was chosen so that there is some consistency in the length of the ID for a long time to come.

In addition, the API response for a need now returns just the ID instead of a full URL to the need.