Fix username typeahead in Remarkup with German keyboard layout

Summary:
Ref T10252. The previous fix rPa8a9fddb0738 only works for macOS.
Under Windows the @ symbol is composed of AltGr+q. For Chrome and Edge the "AltGr" keypressEvent is like pressing the Control key and the Alt key at the same time.
This fix changes the condition in such a way, that this case (pressing Control and Alt at the same time) is not blocked.

Test Plan:
Testing for the issue:

• Launch Windows 10, Select German Keyboard, Use latest Chrome (60)
• Observe typing @ does not trigger typeahead
• Apply patch, retest, see typeahead.

Regression tested:

• Windows 10, Chrome, Firefox, Edge
• Mac OS, Chrome, Firefox, Safari
• Keyboard layouts, English, French, German, Spanish

All tests passed

Reviewers: benwick, epriestley

Reviewed By: epriestley

Subscribers: epriestley

Maniphest Tasks: T10252

Differential Revision: https://secure.phabricator.com/D18269