conversation-viewer: set searchentry width to same as inbox's

Previously our search entry for conversations was too small
for the placeholder text, and looked awkward in comparison
to the entry for searching messages.

Now we set them to have the same width-chars.

Fixes #390
5 jobs for <span class="ref-name">wip/christopherdavis/conversation-search-width</span> in 39 minutes and 16 seconds (queued for 28 seconds)
Status Job ID Name Coverage
  Build
passed #276797
fedora

00:15:26

passed #276798
ubuntu

00:24:00

 
  Test
passed #276799
flatpak

00:11:39

 
  Deploy
passed #276800
review

00:00:33

passed #276801
allowed to fail manual
stop_review

00:00:28