Replace label priority field with a nullable type

The priority field for a project or group label is currently an int64 field in the response structs.

However, it is possible to set the priority explicitly to a number of values that are not currently catered for in the response struct.

This issue proposes replacing int64 with an explicit type to represent all the available options, including null.

For more context, here is the proposal from @PatrickRice

I'd personally prefer to use an explicit nullable type instead of pointers in the future because there are _actually_ 4 different states a response attribute can be in:

* Not included in the response
* Included in the response, explicit null
* Included in the response, explicit empty
* Included in the response, explicit value

and a nullable type would allow us to check all those values instead of just 3 of them (with a pointer, the first 2 can't be differentiated)

And the original discussion issue: https://github.com/xanzy/go-gitlab/pull/1407