Extract "Today" in device list item to string resource

Related to #4752
Closes #4780
// FREEBIE

res/values/strings.xml

@@ -208,6 +208,7 @@
     <string name="DeviceListItem_unnamed_device">Unnamed device</string>
     <string name="DeviceListItem_linked_s">Linked %s</string>
     <string name="DeviceListItem_last_active_s">Last active %s</string>
+    <string name="DeviceListItem_today">Today</string>
 
     <!-- ShareActivity -->
     <string name="ShareActivity_share_with">Share with</string>

src/org/thoughtcrime/securesms/util/DateUtils.java

@@ -87,7 +87,7 @@ public class DateUtils extends android.text.format.DateUtils {
     SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
 
     if (simpleDateFormat.format(System.currentTimeMillis()).equals(simpleDateFormat.format(timestamp))) {
-      return "Today";
+      return context.getString(R.string.DeviceListItem_today);
     } else {
       String format;