Fix horizontal scrollbar in Conversations

Fixes: CNVS-33835

Test plan:
- Log into Canvas as any kind of user: admin, teacher, etc.
- When you go to your Inbox, you should no longer
  see an unnecessary horizontal scrollbar
  (compare with production or beta, which both
  have the scrollbar)

Change-Id: I3763126d0812529db02db041582c0784521fdb18
Reviewed-on: https://gerrit.instructure.com/98191
Tested-by: Jenkins
Reviewed-by: Pam Hiett <phiett@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
QA-Review: Dan Sasaki

Author: Chris Hart

app/stylesheets/pages/conversations/_conversations_new.scss

@@ -8,6 +8,10 @@
   height: 100%;
 }
 
+.ic-Layout-wrapper {
+  position: relative;
+}
+
 .ic-Layout-columns {
   position: absolute;
   top: 0; bottom: 0;