fix overflow with conversations courses

fixes COMMS-1732

Test Plan:
- make 100 courses
- enroll yourself in those courses
- go to the inbox click on the course selection (top left)
- notice you can scroll through them.

Change-Id: I5b6d124b75115028a5c5d97951a74b37ffff80f6
Reviewed-on: https://gerrit.instructure.com/171277
Reviewed-by: Landon Gilbert-Bland <lbland@instructure.com>
Tested-by: Jenkins
QA-Review: Steven Burnett <sburnett@instructure.com>
Product-Review: Steven Burnett <sburnett@instructure.com>

public/javascripts/vendor/bootstrap-select/bootstrap-select.js

@@ -344,8 +344,8 @@
                   } else {
                       minHeight = 0;
                   }
-                  menu.css({'max-height' : menuHeight + 'px', 'overflow' : 'visible', 'min-height' : minHeight + 'px'});
-                  menuInner.css({'max-height' : (menuHeight - menuPadding) + 'px', 'overflow-y' : 'visible'});
+                  menu.css({'max-height' : menuHeight + 'px', 'overflow' : 'auto', 'min-height' : minHeight + 'px'});
+                  menuInner.css({'max-height' : (menuHeight - menuPadding) + 'px', 'overflow-y' : 'auto'});
           }
               getSize();
               $(window).resize(getSize);