fix: address unavailable user display issues [FS-1684, FS-1570]

[V-Gira]

FS-1684 [web] Display fallback UI for user profile when user metadata is not available

FS-1570 [web] Show system message indicating who will receive the message later

Issues found in playtest

• warning message for some unreachable users displays "*empty string*, *empty string* will receive your message later. It should be "2 users from *offline b-e* won't get your message"
• Conversation title for unreachable user is "...", should be "Name not available"
  
• Handles were used in warning message instead of display names

Solutions

• implement logic to add users with id stored in conversation but no session initiated to the list of unreachable users
• replace "..." by "Name not available" for 1 on 1 conversation titles (no change to group conversations)
  
• replace handle by display name for known users
  

[codecov]

Codecov Report

Merging #15159 (a2ff351) into dev (7ded3a7) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev   #15159      +/-   ##
==========================================
+ Coverage   43.15%   43.20%   +0.05%     
==========================================
  Files         644      644              
  Lines       21634    21648      +14     
  Branches     4954     4956       +2     
==========================================
+ Hits         9337     9354      +17     
+ Misses      11105    11102       -3     
  Partials     1192     1192