Redo line-height globally

- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

scss/_dropdown.scss

@@ -70,7 +70,6 @@
   padding: 3px 20px;
   clear: both;
   font-weight: normal;
-  line-height: $line-height;
   color: $dropdown-link-color;
   text-align: inherit; // For `<button>`s
   white-space: nowrap; // prevent links from randomly breaking onto new lines
@@ -147,9 +146,8 @@
 // Dropdown section headers
 .dropdown-header {
   display: block;
-  padding: 3px 20px;
+  padding: 5px 20px;
   font-size: $font-size-sm;
-  line-height: $line-height;
   color: $dropdown-header-color;
   white-space: nowrap; // as with > li > a
 }