Slightly widen index sidebar, overflow buttons properly

First half of #349 fix. Supersedes #734 (190px wide instead of 200px, correctly modify margin-left of .sideNavOffset, more descriptive commit message)

less/lib/Button.less

@@ -192,6 +192,8 @@
 .Button--block {
   display: block;
   width: 100%;
+  overflow: hidden;
+  text-overflow: ellipsis;
 
   // Vertically space out multiple block buttons
   + .Button--block {