Prevent directory listing width from growing > 75%

Fixes CNVS-29059

Test plan:
 - Pull up /courses/<n>/files
 - Make a file or directory with a name that's around 230 characters
 - Ensure the list of files in the directory doesn't expand and cause
 - the directory tree on the left to be squished out of existence

Change-Id: I96029fb60e14ad81678ab92090c3a5270d19871d
Reviewed-on: https://gerrit.instructure.com/78811
Tested-by: Jenkins
Reviewed-by: Steven Burnett <sburnett@instructure.com>
QA-Review: Heath Hales <hhales@instructure.com>
Product-Review: Alex Boyd <aboyd@instructure.com>

Author: javawizard

app/stylesheets/pages/react_files/_react_files.scss

@@ -172,6 +172,9 @@ body:not(.with-left-side) & .ic-app-nav-toggle-and-crumbs--files {
   position: relative;
   margin: 10px;
   flex: 3;
+  // long file names cause this container, of all things, to expand unreasonably even after they get elided. Work
+  // around that for the moment with this.
+  max-width: 75%;
   display: flex;
   flex-direction: column;