Previously we hardcoded the DOWNLOAD_URL_EXPIRES_AFTER_SECONDS const
inside S3Helper to be 5 minutes (300 seconds). For various reasons,
some hosted sites may need this to be longer for other integrations.
The maximum expiry time for presigned URLs is 1 week (which is
604800 seconds), so that has been added as a validation on the
setting as well. The setting is hidden because 99% of the time
it should not be changed.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
