Revert "Images are responsive by default"

This reverts commit d29f851e82.

Making <img>s responsive by-default without opt-in can severely break
third-party widgets such as Google Maps.
This was an acknowledged problem in Bootstrap v2
(see https://github.com/twbs/bootstrap/issues/1506 ) and was fixed in v3
by requiring the .img-responsive class for explicit opt-in
(see 09cdee2f03).
The situation hasn't really changed since then. The responsive-by-default
approach hasn't become any more suitable in the intervening time.

So let's avoid having this regress in v4. :-)

scss/mixins/_image.scss

@@ -1,3 +1,19 @@
+// Image Mixins
+// - Responsive image
+// - Retina image
+
+
+// Responsive image
+//
+// Keep images from scaling beyond the width of their parents.
+
+@mixin img-responsive($display: block) {
+  display: $display;
+  max-width: 100%; // Part 1: Set a maximum relative to the parent
+  height: auto; // Part 2: Scale the height according to the width, otherwise you get stretching
+}
+
+
 // Retina image
 //
 // Short retina mixin for setting background-image and -size.