Fix Source control username preference UI
Fix Source control username preference UI

The layout of 'Source Control Username' field in Account Preferences was
broken for Modern UI.

Fixes #180

Signed-off-by: Damien Regad <d9b599957bd54c22143053c16557076ca2185dbd@mantisbt.org>

make-all/source-integration,mantisbt-plugins/source-integration,mantisbt-plugins/source-integration,make-all/source-integration,make-all/source-integration,dregad/source-integration,dregad/source-integration,dregad/source-integration,mantisbt-plugins/source-integration