Remind sources to check back for replies after first submission

Along with the existing notification from the "flag for reply" flow
(mentioned in https://github.com/freedomofpress/securedrop/issues/683#issuecomment-65484457),
this resolves #683.
Merge pull request #794 from freedomofpress/remind-to-check-for-replies-after-first-submission

Remind sources to check back for replies after first submission