perf: add latest release branch as option in branch drop-down

This was requested by the tools folks because it allows creating stable
links without knowing the magic query parameter.

Change-Id: I29570f70cf89957a54a09d9eb94c45ca1fb8a283
Reviewed-on: https://go-review.googlesource.com/c/build/+/613916
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

perf/app/dashboard/index.html

@@ -480,6 +480,10 @@ fetch('./formfields.json')
 			opt.innerText = formFields.Branches[i];
 			select.appendChild(opt);
 		}
+		var opt = document.createElement('option');
+		opt.value = "latest-release";
+		opt.innerText = "Latest release branch";
+		select.appendChild(opt);
 		prefillSearch(formFields);
 	});