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/mixins/_buttons.scss

@@ -95,6 +95,5 @@
 @mixin button-size($padding-y, $padding-x, $font-size, $line-height, $border-radius) {
   padding: $padding-y $padding-x;
   font-size: $font-size;
-  line-height: $line-height;
   @include border-radius($border-radius);
 }

scss/mixins/_pagination.scss

@@ -4,7 +4,6 @@
   .page-link {
     padding: $padding-y $padding-x;
     font-size: $font-size;
-    line-height: $line-height;
   }
 
   .page-item {

scss/mixins/_reset-text.scss

@@ -5,7 +5,7 @@
   font-weight: normal;
   letter-spacing: normal;
   line-break: auto;
-  line-height: $line-height;
+  line-height: $line-height-base;
   text-align: left; // Fallback for where `start` is not supported
   text-align: start;
   text-decoration: none;