Mark unread submissions

[redshiftzero]

This PR is to merge in the work of @mdrose from #1358 which adds in an unread icon on the submissions view for submissions that are unread as well as the option to download all unread messages from the submission view. I broke out the getting unread submissions functionality and added a couple unit tests for it. Let me know if other tests need writing and I will add! Closes #1353.

[psivesely]

1. I don't think we need the extra Python functions to accomplish this webapp functionality. Why not just use the doc.downloaded attribute? (As a side note, I'm not a fan of how inconsistent naming is in the application--I'd prefer if object references to a Submission or array-type of Submissions always used the variable name "submission" or "submissions," unless there is some specific contextual reason not to.)
2. I think instead of omitting an icon in the case a message has not been received, an open envelope should be included. The empty space feels inconsistent, and the columns shift when the page is refreshed after reading all unread messages, which I think looks poor.
3. Similarly, there is now this absent space in rows for outgoing messages. That got me thinking: why not also mark sent messages as read if the source has logged in since they were sent? Since we already expose the rough last login time of each source to the journalists, we might as well inform them if the source has read (or at least had the opportunity to read) their replies. Font Awesome includes envelope-o and envelope-open-o icons, which would help reinforce the visual distinction between replies and submissions.

[redshiftzero]

1. Good point! Fixed this.
2. Note this icon you recommended looks like it was just added in Font Awesome 4.7, so I’ve updated Font Awesome and this now uses this icon to denote the submissions that have been read.
3. We’re not actually storing the timestamp of the reply as far as I can tell anywhere in the database - we do have the last_access column in the journalists table and last_updated in the sources table, but as soon as a journalist logs in then we are updating last_access. Given that sources are instructed to delete their replies when they read them, and that this is not as straightforward as showing an icon for doc.downloaded (unless I’m overlooking something), I think we should table this until it’s requested. However, to address the aesthetic concern I’ve aligned the replies so that they’re flush with the messages.

Also rebased this on develop. Let me know your thoughts @fowlslegs!

[redshiftzero]

Screenshot of how this looks now:

[psivesely]

1. So Source.last_updated only gets bumped their key pair is generated via source.async_genkey, or when they make a submission. This is a reasonable choice to help protect source metadata. So if you see that a source has made a new submission, this kind of obviates the need for some other visual indication that they (may) have read your message. So I guess I'm now in favor of how you've got it.

[psivesely]

Nice work!