LPS-44257 Portal dockbar appears in two lines on mobile devices

Hey @Robert-Frampton,

Attached is an update for https://issues.liferay.com/browse/LPS-44257.

The .brand styling in dockbar.css has been moved to the control panel css files. I also fixed an error and an outdated syntax in the flexbox mixins. Please let me know if there is any issue. 

Thanks!