Move blockquote footer font size to variables

Blockquote footer font size is currently hardcoded to 80%, implying (in a comment) that this would result in the default font size. However, since the blockquote font size itself is variable, this is not necessarily true, and 80% of anything actually results in an arbitrary font size.

80% as a default is still fine.

scss/_type.scss

@@ -116,7 +116,7 @@ mark,
 
 .blockquote-footer {
   display: block;
-  font-size: 80%; // back to default font-size
+  font-size: $blockquote-small-font-size;
   color: $blockquote-small-color;
 
   &::before {

scss/_variables.scss

@@ -298,6 +298,7 @@ $small-font-size:             80% !default;
 $text-muted:                  $gray-600 !default;
 
 $blockquote-small-color:      $gray-600 !default;
+$blockquote-small-font-size:  $small-font-size !default;
 $blockquote-font-size:        ($font-size-base * 1.25) !default;
 
 $hr-border-color:             rgba($black, .1) !default;