Fixes Profile avatar on thumbnail no longer showing correct image

Thumbnails will now have namespace column which will be populated with
its attachments namespace on save. Old thumbnails will have null
namespace and should behave as they used to. New thumbnails will work
like boss.

Fixes CNV-12380

Test Plan
=========

Prerequisites
-------------

- S3 storage
- **Sharding**
	- multiple users with the same ID on different shards
	- profile photos on all the users with the same fileID

Details
-------

Before the fix the last uploaded avatar will be the *thumbnail* for
all user avatars that collide with each other.
After the code is updated all avatars will be blank.
After `bundle exec rake db:migrate` avatars should display correctly.

Change-Id: Iec533086308da352f317cda60b404327b14516e3
Reviewed-on: https://gerrit.instructure.com/39108
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>