Add a SRKW option to the Orca labeling UI

In some cases, an annotator recognizes that a call is from a Southern Resident Killer Whale (SRKW), but isn't sure which pod was the source (J, K, or L). Add a "S" icon to the existing choices (J, K, L, and B for Bigg's). When user mouses over this new "S" icon, the explanation modal says "SRKW" just as the B icon currently says "Bigg's".