splash-url-exclusion: Exclude '/heartbeat' from redirect

Update the dependency to load this commit:
https://github.com/edx-solutions/django-splash/commit/6e8bd685214aebe78d0cfdf5572e27c5f1bc357a

Which allows to prevent `/heartbeat` from being redirected, which is
necessary on production servers
ImageExplorer MVP implementation

update hotspot reveal styling

address some PR feedback. Start to build out some Jasmine tests

address PR comments

add some default content
fix when running under https