Plural group join message

Closes #4535
Fixes #4521
// FREEBIE

res/values/strings.xml

@@ -690,7 +690,10 @@
     <string name="MmsPreferencesFragment__invalid_host">The text entered was not a valid host</string>
 
     <!-- GroupUtil -->
-    <string name="GroupUtil_joined_the_group">%1$s joined the group.</string>
+    <plurals name="GroupUtil_joined_the_group">
+        <item quantity="one">%1$s joined the group.</item>
+        <item quantity="other">%1$s joined the group.</item>
+    </plurals>
     <string name="GroupUtil_group_updated">Group updated.</string>
     <string name="GroupUtil_group_name_is_now">Group name is now \'%1$s\'.</string>
 

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

@@ -74,7 +74,8 @@ public class GroupUtil {
       String        title       = groupContext.getName();
 
       if (members != null) {
-        description.append(context.getString(R.string.GroupUtil_joined_the_group, members.toShortString()));
+        description.append(context.getResources().getQuantityString(R.plurals.GroupUtil_joined_the_group,
+                members.getRecipientsList().size(), members.toShortString()));
       }
 
       if (title != null && !title.trim().isEmpty()) {