CYB-368 - Gallery: browser back button in item detail view doesn't return to gallery main

https://jira.ets.berkeley.edu/jira/browse/CYB-368
@nicolaasmatthijs since that wasn't in master yet, it didn't make sense to add it. I created https://jira.ets.berkeley.edu/jira/browse/CYB-370 as my follow up task on this PR.