Feature: Unread posts indicator

[FelberMartin]

Problem Description

When navigating to a chat with unread posts, the users had no clear indicator where the new unread posts begin. This PR adds an unread indicator inspired by slack and ls1intum/Artemis#10018

Changes

• Added unread indicator
• Added index to paged posts to correctly display the unread indicator

Steps for testing

• Go to any chat with unread posts
• See that a visual indicator separates read from unread posts

Screenshots

Follow-up

Known bug:
When the unread counter in the ConversationOverview increases due to a websocket update (the user is currently in the communication tab and a new post in a chat that is not open is sent), then the unread indicator is not displayed. This is due to the unread counter only being updated in the ConversationOverviewViewmodel. This bug will be tackled with #394

[eylulnc]

Code looks good.
Works as described, I did not encounter any problem other than the one mentioned in the description.

[julian-wls]

I just tested it and it works fine most of the time. I got two concerns though:
Regarding the bug you mentioned my opinion would be to release this feature only if the bug is fixed. I think you can merge it now but not release it until we tackled this bug. I experienced this bug a couple of times and found it quite confusing for users.
Another problem I experienced was the following:

x.mp4

I just wanted to mention it here, but I think it is server related as the channels sometimes don't update. When going back to the channel the unread indicator was also still there.

And one comment (Everything else lgtm):

[FelberMartin]

I also experienced the second bug that you mentioned, I think this one is (as you mentioned) server related and also occurs on develop. This bug however could also be mitigated by storing the conversations locally.

Overall I agree with you that the resulting behavior of these two bugs are quite disturbing, so I will merge this PR now, but include a feature toggle that deactives the feature for now. When the bugfixes are introduced, we then can also remove the feature toggle again.

I also updated #394 to keep track of the mentioned TODOs

[julian-wls]

I agree. Lgtm now.