Make "Contact joined Signal" translatable

Closes #6534
// FREEBIE

res/values/strings.xml

@@ -1200,6 +1200,7 @@
     <string name="preferences_chats__show_invitation_prompts">Show invitation prompts</string>
     <string name="preferences_chats__display_invitation_prompts_for_contacts_without_signal">Display invitation prompts for contacts without Signal</string>
     <string name="preferences_chats__message_text_size">Message font size</string>
+    <string name="preferences_events__contact_joined_signal">Contact joined Signal</string>
 
     <!-- **************************************** -->
     <!-- menus -->

res/xml/preferences_notifications.xml

@@ -67,7 +67,7 @@
     <PreferenceCategory android:title="@string/preferences_notifications__events">
         <org.thoughtcrime.securesms.components.SwitchPreferenceCompat
             android:key="pref_enable_new_contacts_notifications"
-            android:title="Contact joined Signal"
+            android:title="@string/preferences_events__contact_joined_signal"
             android:summary="@string/preferences__show_a_notification_for_new_signal_contacts"
             android:defaultValue="true" />
     </PreferenceCategory>