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/_tables.scss

@@ -10,7 +10,6 @@
   th,
   td {
     padding: $table-cell-padding;
-    line-height: $line-height;
     vertical-align: top;
     border-top: $table-border-width solid $table-border-color;
   }