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)

docs/assets/scss/_content.scss

@@ -20,7 +20,6 @@
         > th,
         > td {
           padding: $table-cell-padding;
-          line-height: $line-height;
           vertical-align: top;
           border: 1px solid $table-border-color;