feat: add pin button to custom local timelines

This is now a thing due to the new instance about fragment, where you can see a server's local timeline from there, and from there you should be able to pin it. This commit addresses that

mastodon/src/main/res/menu/custom_local_timelines.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<menu xmlns:android="http://schemas.android.com/apk/res/android">
+    <item
+        android:id="@+id/pin"
+        android:title="@string/sk_pin_timeline"
+        android:icon="@drawable/ic_fluent_pin_24_regular"
+        android:showAsAction="always" />
+</menu>