Create AuthoredHintViewRequests only if current_user.

[ashercodeorg]

On 2017-02-09, it was discovered that we have been creating AuthoredHintViewRequest records for all users, signed-in or non-signed-in. This PR makes it so that we only create records for signed-in users, the desired behavior.

TODO(asher, hamms) (here and in follow-up PRs):

• (hamms) Fix the client-side logic to stop making controller requests for non-signed-in users.
• (hamms) Update the controller and test to return a :bad_request rather than :accepted for non-signed-in users.
• (asher) Remove all AuthoredHintViewRequest data from the DB associated with non-signed-in users.
• (asher) Make AuthoredHintViewRequest.user_id a non-nullable column.

Also includes readability and style cleanup.

[codecov-io]

Codecov Report

Merging #13179 into staging will not change coverage.

@@           Coverage Diff            @@
##           staging   #13179   +/-   ##
========================================
  Coverage    58.01%   58.01%           
========================================
  Files          711      711           
  Lines        52109    52109           
  Branches      1154     1154           
========================================
  Hits         30232    30232           
  Misses       21672    21672           
  Partials       205      205

Flag	Coverage Δ
#integration	49.78% <100%> (ø)	✅
#unit	41.35% <100%> (ø)	✅

Impacted Files	Coverage Δ
apps/src/StudioApp.js	59.52% <100%> (ø)	✅

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[ashercodeorg]

LGTM your commits, @Hamms.