fix(message-list): side-bar button shouldn't be selectable

Summary: fixes T2852

Test Plan: tested manually

Reviewers: bengotow

Reviewed By: bengotow

Maniphest Tasks: T2852

Differential Revision: https://phab.nylas.com/D1859

internal_packages/message-list/stylesheets/message-list.less

@@ -54,6 +54,7 @@
   color: @text-color-subtle;
   margin-left: @spacing-standard;
   cursor:default;
+  -webkit-user-select: none;
   .img-wrap {
     margin-right: @spacing-half;
     position: relative;