Let participants letter filtering have a @ like glossary has

It would be great if you could add an "@" to the participant letter filtering. As in the glossary, you should be able to find all users whose name starts with something other than a-z, like "Österreicher" or "Åkerlund" under "@". 

With this improvement we would not have such names incorrectly under "O" and "A" or not a separate filter for each Umlaut, etc.