user: strip leading @ from user query when searching for users if present

closes #117

lib/pleroma/user.ex

@@ -375,6 +375,9 @@ def get_recipients_from_activity(%Activity{recipients: to}) do
   end
 
   def search(query, resolve) do
+    # strip the beginning @ off if there is a query
+    query = String.trim_leading(query, "@")
+
     if resolve do
       User.get_or_fetch_by_nickname(query)
     end