Improve copy of start/end segment button

public/_locales/en/messages.json

@@ -77,10 +77,10 @@
         "message": "No segments found"
     },
     "sponsorStart": {
-        "message": "Segment Starts Now"
+        "message": "Start Segment Now"
     },
     "sponsorEnd": {
-        "message": "Segment Ends Now"
+        "message": "End Segment Now"
     },
     "sponsorCancel": {
         "message": "Cancel Creating Segment"

public/popup.css

@@ -377,7 +377,7 @@
 }
 
 /*
- * Generic buttons used for "Segment Starts Now" and "Submit Times"
+ * Generic red buttons used for "Start Segment Now", "Submit Times" etc.
  */
 .sbMediumButton {
   border: none;