Ability to mark individual messages as read in the inbox

[folkemat]

Hi, this PR implements the “read_message” function of the Reddit API, which allows to mark individual messages as read.
A message that is tapped on in the inbox now gets individually marked as read.
I have disabled this option by default, but maybe it's not a big stretch to enable it by default.

Closes #554

[frozenpandaman]

Really looking forward to this! Hope it can be merged soon… just want to say that there's lots of support and excitement for this, @QuantumBadger! :)

See also https://old.reddit.com/r/RedReader/comments/1t3jmej/why_does_the_inbox_behave_so_inefficiently/

[QuantumBadger]

Thanks for the contribution @folkemat. I had a look through this PR and the main issue is that there's no visual indicator for whether a message is read or not, so if someone taps a message then nothing will appear to have happened.

Rather than merge this, I've gone ahead and implemented a larger set of improvements to the inbox, which show the read status of each post and also add a "mark read" button:

2d9613d

This is present in Alpha 372:

https://redreader.org/alpha/

Let me know if you encounter any issues!