Check for whether to mark messages read on mounted

[nicksellen]

Fixes https://community.karrot.world/t/marking-messages-as-read-in-the-sidebar/1233

(I was going to fix a bunch of little bugs, but there was only one I did, the other was a backend one...)

This just runs some of the logic to decide whether to mark as read when we first open the conversation, so we don't get hanging unread messages, as described in the link.

[codecov[bot]]

Codecov Report

Merging #2660 (d44a863) into master (6b7f9b0) will decrease coverage by 0.01%. The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master    #2660      +/-   ##
==========================================
- Coverage   75.86%   75.86%   -0.01%     
==========================================
  Files         384      384              
  Lines       41013    41006       -7     
  Branches     2340     2356      +16     
==========================================
- Hits        31116    31109       -7     
  Misses       9897     9897              

Impacted Files	Coverage Δ
src/messages/components/ChatConversation.vue	68.20% <93.33%> (-0.57%)	:arrow_down:

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