MDL-56763 theme_boost: ignore boostrap popover rtl scss

Because Bootstrap Javascript does is not expecting to be flipped, it breaks the popover on RTL languages.
We already swap the directions on the help icon template so popover scss rules can be ignored.

theme/boost/readme_moodle.txt

@@ -10,6 +10,8 @@ The bootstrap repository is available on:
 https://github.com/twitter/bootstrap.git
 
 To update to the latest release of twitter bootstrap:
+* re-apply /* rtl:begin:ignore */ on the top of _popover.scss before .popover rule and /* rtl:end:ignore */ before
+  .popover-arrow::after rule. See MDL-56763 commit (1a4faf9b).
 * remove all files from scss/bootstrap,
 * download the new scss files and store them in scss/bootstrap
 * update ./thirdpartylibs.xml

theme/boost/scss/bootstrap/_popover.scss

@@ -1,3 +1,4 @@
+/* rtl:begin:ignore */
 .popover {
   position: absolute;
   top: 0;
@@ -141,6 +142,7 @@
 .popover-arrow {
   border-width: $popover-arrow-outer-width;
 }
+/* rtl:end:ignore */
 .popover-arrow::after {
   content: "";
   border-width: $popover-arrow-width;