blobs_helper.rb : A student can get the URL of their snapshot image.

[#44420837]
https://www.pivotaltracker.com/story/show/44420837

A student can get the URL of their snapshot image and use it in the drawing tool or Wallwisher.
Need a publicly available URL so that can get to it without logging in.
schema checkin… (after migration from upstream changes…)
Merge remote-tracking branch 'origin/master' into remove-current-visitor-instance-var

need to keep this branch up-to-date as it is long running
don't dump the schema here

This will dump the current cucumber db schema which probably
hasn't been migrated yet. This task should assume the schema is
up-to-date just like the normal db:test:prepare task does