Merge pull request #208 from coyote/sql_get_gallery_data

CYB-347 Select image fields for Gallery API
CYB-348 author_canvas_user_id & created_at should be integers instead of strings

  * numerizes those fields in comments
https://jira.ets.berkeley.edu/jira/browse/CYB-348
Merge pull request #209 from coyote/numerize_comment_fields

CYB-348 author_canvas_user_id & created_at should be integers instead of strings
CYB-352 Return DB connections to the pool after done with using raw SQL

 * use a block yielding to the execution statement so that the conneciton is only used
   within said block
 * decrease sidekiq concurrency, so fewer connections are used at once.

https://jira.ets.berkeley.edu/jira/browse/CYB-352