Break out ConversationMessage link popover into seperate UI file

This reduces the number of widgets to be parsed and constructed when
showing a conversation, improving loading perf a bit for large
conversations.

See #230
5 jobs for mjog/conversation-message-link-popover in 22 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #512458
fedora

00:12:30

passed #512459
ubuntu

00:03:35

 
  Test
passed #512460
flatpak

00:09:08

 
  Deploy
passed #512461
review

00:00:34

passed #512462
allowed to fail manual
stop_review

00:00:43