UX: Admins should only see their own PMs when searching in:all

Admins are technically allowed to access all PMs, but it can be confusing to include them all in search. Follow-up to e0605029dc

lib/search.rb

@@ -828,7 +828,7 @@ class Search
        end
     elsif opts[:type_filter] === "all_topics"
       private_posts = posts.where("topics.archetype = ?", Archetype.private_message)
-      private_posts = private_posts.private_posts_for_user(@guardian.user) unless @guardian.is_admin?
+      private_posts = private_posts.private_posts_for_user(@guardian.user)
 
       posts = posts.where("topics.archetype <> ?", Archetype.private_message).or(private_posts)
     else

spec/components/search_spec.rb

@@ -348,11 +348,11 @@ describe Search do
                                 guardian: Guardian.new(u3))
         expect(results.posts.length).to eq(1)
 
-        # Admin
+        # Admin doesn't see private topic
         results = Search.execute('spam',
                   type_filter: 'all_topics',
                   guardian: Guardian.new(u4))
-        expect(results.posts.length).to eq(2)
+        expect(results.posts.length).to eq(1)
 
         # same keyword for different users
         results = Search.execute('ham',