Ensure participation has a primary key.

For some reason our participations model has lost its primary
key on staging.

[ci skip]
Add database.yml copy stuff to README
Refactor DownloadManuscriptWorker
`include RestrictAccess` in IhatJobsController
Set headers in spec/controllers/ihat_jobs_controller_spec
Merge pull request #508 from Tahi-project/download_manuscript_worker_refactoring

Refactor DownloadManuscriptWorker
Let it breathe
Get comments from the store by filter.
Unbind user thumbnail helper
Add ETahi.computed.limit arrayComputed helper.
Sort comments using arrayComputed stuff.

Prevent rerendering when possible.
Reload comment instead of task from eventstream.

Meta for a comment now just reloads the comment rather than the
whole task.  We found out the comment#show route was broken so
we fixed it as well.
Nobody needs commentLooks they've already read.
Make ETahi.computed.limit more robust.
Show reviewer papers on dashboard.
Use commentLooks on the dashboard.
Remove unreadCommentsCount from LitePaper
Fix key dependencies for unread comments count
get rid of shownComments in the commentsBoard.
Add a standard authorization response