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/_images.scss

@@ -17,7 +17,6 @@
 // Image thumbnails
 .img-thumbnail {
   padding: $thumbnail-padding;
-  line-height: $line-height;
   background-color: $thumbnail-bg;
   border: $thumbnail-border-width solid $thumbnail-border-color;
   border-radius: $thumbnail-border-radius;