Issue 4528: Indicate what labels a reviewer can vote on

Affected Version: 2.13-rc0-312-g9b9adbf

What steps will reproduce the problem?
1. View any change (e.g., https://gerrit-review.googlesource.com/c/85291/)
2. Mouse over a reviewer

What is the expected output?
Name, email, and a list of labels they can vote on.

What do you see instead?
Name and email.

Please provide any additional information below.
I'm not attached to tooltips as a way of doing this --- that's just what the GWT UI does.

https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#detailed-labels lists what labels each reviewer can vote on.

I'm more interested in seeing who can *approve* (and not just +1) for each label type, but that's not exposed through the REST API yet. I filed this so that polygerrit can be ready once that happens.