CLC-1039 Click banner logo to return to dashboard

https://jira.media.berkeley.edu/jira/browse/CLC-1039
    
Also fixes CLC-1040 Masthead at mobile resolution
https://jira.media.berkeley.edu/jira/browse/CLC-1040
    
Logo image now rides on a CSS background and scales to 100% of 2-columns mobile. 
To accomplish this, the small launcher icons (bdrive, etc.) are hidden at small screen sizes.