Trigger autocomplete when "@" is typed on German keyboards

Summary:
Ref T10252. On the German keyboard layout, you must type "Alt" + "L" to generate an "@" character.

We currently ignore this event, assuming it's a keyboard command. However, I think we can safely continue so that autocomplete works on German layouts.

Test Plan:

• Switched keyboard layout to German.
• Typed Alt + L to generate an "@".
• Typed some username text.
• Got autocompleter.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10252

Differential Revision: https://secure.phabricator.com/D16259