[WIP] Add last active to the PM window, and add more features to PMs.

[plugyawn]

What does this PR do?
Aims to add a few more features to the PM window, such as adding a last active message.

Tested?

• Manually
• Existing tests (adapted, if necessary)
• New tests added (for any new behavior)
• Passed linting & tests (each commit)

Commit flow

• added the last active message.

Notes & Questions
What more could we add to the PM Window?

Interactions

Visual changes

[neiljp]

@plugyawn I'd recommend discussing this on the stream before going too far into this.

The equivalent detail we've discussed previously would be a summary of this via an extension of the message list version which shows the user presence markers (which is not in the web app). The web app has that in the autocomplete/typeahead equivalent, though not in the compose box itself during the rest of composing of PMs. That would basically be part of #1155 for the summary data for what you're looking at here.

[plugyawn]

Okay!

The last seen thing was something that was commonplace on messenger applications, and it's a pretty handy indicator to have, so I guessed it's useful to have it here, too.

Also attaching an image for general reference.

[zulipbot]

Heads up @plugyawn, we just merged some commits that conflict with the changes you made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.