Add docs for read/unread, updated screenshots w/ new source list layout

[eloquence]

SecureDrop Client 0.4.0 ships initial support for seen/unseen at the source level (sources with submissions unseen by any journalist user are bolded). It also changes the layout of the source list to be more compact. We should document the seen/unseen feature, and update our screenshots.

[eloquence]

Probably best done alongside other screenshot updates, e.g., #61, and updated screenshots in our training slide deck.

[eloquence]

Not exhaustive yet, but here's a screenshot that shows just seen/unseen (bolded sources vs. unbolded ones):

And here's one with a document and a conversation selected.

[eloquence]

Updater flow for good measure (since the Whonix icons have changed relative to what's in the docs):

[ninavizz]

Lookit those swell new Whonix icons!

[eloquence]

See also freedomofpress/securedrop-workstation#719 for other needed screenshot updates.

[eloquence]

OK, I've taken a whole new set of screenshots. This time I renamed the system user account to nbly first, as well. I'll start putting these into the docs in the next couple of days and then will fill in any remaining gaps.