Replace help icon with lighter variant.

- Add `height` rule to `top-bar__btn` to help with alignment

src/images/icons/help.svg

-<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
-  <path fill="currentColor" fill-rule="nonzero" d="M8 16A8 8 0 1 1 8 0a8 8 0 0 1 0 16zm-.5-5.805c.496 0 .768-.29.873-.788.091-.623.3-.943 1.23-1.515.987-.616 1.497-1.381 1.497-2.503C11.1 3.66 9.765 2.5 7.779 2.5c-1.503 0-2.622.631-3.07 1.612-.14.282-.209.564-.209.884 0 .564.343.928.895.928.426 0 .741-.208.916-.683.224-.661.685-1.018 1.37-1.018.77 0 1.3.505 1.3 1.233 0 .683-.272 1.055-1.174 1.627-.825.512-1.251 1.092-1.251 1.968v.104c0 .609.35 1.04.943 1.04zm.013 3.305c.637 0 1.147-.512 1.147-1.166 0-.654-.51-1.166-1.147-1.166-.629 0-1.14.512-1.14 1.166 0 .654.511 1.166 1.14 1.166z"/>
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16" aria-hidden="true" focusable="false" class="Icon Icon--help"><g fill-rule="evenodd"><path fill="none" fill-rule="evenodd" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 4c0-1.657 1.79-3 4-3s4 1.343 4 3-1.79 3-4 3m0 0v2-2zm0 8a1 1 0 1 1 0-2 1 1 0 0 1 0 2z"></path><rect fill="none" stroke="none" x="0" y="0" width="16" height="16"></rect></g></svg>

src/styles/sidebar/components/top-bar.scss

@@ -62,10 +62,11 @@
 .top-bar__btn {
   @include reset-native-btn-styles;
 
+  height: 100%;
   color: $gray-light;
   display: inline-block;
   cursor: pointer;
-  padding: 0 3px;
+  padding: 1px 3px 0 3px;
 
   &:hover {
     color: $gray-dark;
@@ -93,12 +94,6 @@
   user-select: none;
 }
 
-/* FIXME: Temporary fix to make 'help' icon align with other top-bar icons */
-.top-bar__help-icon {
-  width: 15px;
-  height: 15px;
-}
-
 .top-bar__apply-icon {
   display: inline-block;
   line-height: 12px;