CYB-80: Gallery is now integrated into Canvas, fully styled

https://jira.ets.berkeley.edu/jira/browse/CYB-80
https://jira.ets.berkeley.edu/jira/browse/CYB-72

Bug: The Previous and Next buttons do not work yet

@ericayin 
@ericayin is currently working on another part of the Gallery at the moment; can we put this in another PR?