Allow REST API exclude M2M fields

[lampwins]

To support cases where many to many fields degrade performance of API responses (even in depth 0 cases), introduce support to ignore/exclude M2M fields in the API response.

So an API consumer can specify ?exclude_m2m=true and not get any M2M fields at all in the response. This is particularly needed in problem areas like VRF where there are several M2M relationships and frequent opportunity for data scale. The API select2 widget will generally benefit from this across the board.