Fix initial scroll of unread messages marker

[PVince81]

For the initial scroll to work, the first call to getOldMessages must
fetch messages beyond the read marker itself. Otherwise the marker will
appear at the bottom of the screen as there aren't any new messages yet
until lookForNewMessages runs.

Fixes #5872

Tests with conversation that has a lot of new unread messages:

• marker at top of viewport when switching to conversation after an initial page load
• marker at top of viewport when switching back to a conversation after having been in the conversation before

[PVince81]

with a bit of luck this change will also make conversation switching feel even faster because now the new unread messages are loaded right away in the first call instead of waiting for the second one.
it also means one less server call: instead of "getOldMessages", "lookForNewMessages->returns directly", "lookForNewMessages->long poll" it becomes "getOldMessages", "lookForNewMessages->long poll"

[nickvergessen]

PRs against master are now "23"

[nickvergessen]

/backport to stable22

[PVince81]

temp test plan while still debugging, I've pushed a WIP with log statements.

1. Create two conversations "first" and "second" as Alice and invite Bob in both
2. Open converstion "second" as Alice
3. Open a second browser for Bob
4. As Bob, open "first"
5. Quickly type messages: "---", 1, 2, 3, 4, and count up to 20 or more, one number per message
6. As Alice while still on "second", refresh the page
7. As Alice, open "first": this tests the initial load and scroll
8. Wait for unread marker to appear above the "---" message: the viewport should be scrolled so that the marker is visible
9. As Alice, Switch to "second" again
10. As Bob, type another set of messages "---", 1, 2, 3, 4, ... 20
11. As Alice, wait for the conversation list to refresh and display the unread messages counter
12. As Alice, switch to "first" again: this tests the scroll when we already had old messages in the store
13. Wait for unread marker to appear above the "---" message: the viewport should be scrolled so that the marker is visible
14. Scroll down a tiny bit but not to the bottom
15. Create and switch to a new tab in Alice's browser to trigger the blur/focus event, wait for two seconds
16. Go back to the former tab and wait two seconds
17. Read marker should have moved down to the top of the viewport a few messages further down from the previous marker position: this tests the read marker movement when switching away
18. Scroll down to bottom
19. Go back to the empty second tab and wait two seconds
20. Go back to the former tab from Alice and wait two seconds
21. Read marker should have disappeared, the conversation is marked as read: this tests read marker disappearance when scrolled to bottom

[PVince81]

The new approach seems to work but has a tiny glitch: the initial conversation load has the read marker at the bottom and it stays there until lookForNewMessages finishes. I think we had that already in previous iterations and might not be that bad.
If bad we could either try to:

• add a loading spinner below
• or add a spinner on the whole viewport
• and/or smooth scroll to the new marker position after its update

This delay might be annoying if someone scrolls up manually while the code is between getOldMessages and lookForNewMessages, because then it will force-scroll back to the read marker. In theory we could detect those scrolls and cancel the automatic scroll then. Seems a bit overcomplicated though.