Update proxy to route all wp/ requests and load assets through proxy

Avoids cross-origin issues with video, etc.
Add directives and utilities for capturing and displaying video screenshots
Build out 1-up and 2-up versions of streetview slide
Add a gulp server option using express and no livereload
Update video URL to be retrieved via a method of ShotService
Refactor screenshot handling to avoid loading shot video more than once

All screenshots now come from the main shot video, since loading video assets is
expensive just to take screenshots. Anything that needs a screenshot can request one
for a specific timecode and it will get it eventually, with a screenshot for another
timecode in the meantime as a fallback.
Adjust end slide keyframe to fade in