MDL-42195 Bootstrap: Make bootstrap responsive images "opt-in".

This responsive image behaviour prevents setting of image sizes in text editors,
as well as a string of pix icon bugs / profile image bugs across Moodle.

We are switching to Bootstrap 3 syntax where you need img-responsive class to images
that want this behaviour.

theme/bootstrapbase/less/bootstrap/reset.less

@@ -76,7 +76,10 @@ sub {
 // Img border in a's and image quality
 // -------------------------
 
-img {
+// Backport of syntax from Bootstrap 3.
+// Images are not responsive by default - you must add img-responsive class to get this behaviour.
+// MDL-42195 (img changed to img.img-responsive)
+img.img-responsive {
   /* Responsive images (ensure images don't scale beyond their parents) */
   max-width: 100%; /* Part 1: Set a maxium relative to the parent */
   width: auto\9; /* IE7-8 need help adjusting responsive images */

theme/bootstrapbase/readme_moodle.txt

@@ -10,6 +10,7 @@ https://github.com/twitter/bootstrap.git
 To update to the latest release of twitter bootstrap:
 * remove all files from less/bootstrap,
 * download the new less files and store them in less/bootstrap
+* Apply change in MDL-42195 (We don't want responsive images by default).
 * regenerate files using recess: recess --compile --compress moodle.less > ../style/moodle.css **
 * regenerate files using recess: recess --compile --compress editor.less > ../style/editor.css **
 * update ./thirdpartylibs.xml