MDL-86953 blocks: constrain block controls menu on narrow screens.

Extension of work in 93cadaba that constrained the dropdown menu,
we now set a breakpoint to account for overflowing when narrow.

theme/boost/scss/moodle/blocks.scss

@@ -32,6 +32,9 @@
     }
     .dropdown-menu {
         max-width: 500px;
+        @include media-breakpoint-down(sm) {
+            max-width: 85vw;
+        }
         .dropdown-item {
             @include text-truncate;
         }

theme/boost/style/moodle.css

@@ -29604,6 +29604,11 @@ img.icon {
 .block .block-controls .dropdown-menu {
   max-width: 500px;
 }
+@media (max-width: 575.98px) {
+  .block .block-controls .dropdown-menu {
+    max-width: 85vw;
+  }
+}
 .block .block-controls .dropdown-menu .dropdown-item {
   overflow: hidden;
   text-overflow: ellipsis;

theme/classic/style/moodle.css

@@ -29604,6 +29604,11 @@ img.icon {
 .block .block-controls .dropdown-menu {
   max-width: 500px;
 }
+@media (max-width: 575.98px) {
+  .block .block-controls .dropdown-menu {
+    max-width: 85vw;
+  }
+}
 .block .block-controls .dropdown-menu .dropdown-item {
   overflow: hidden;
   text-overflow: ellipsis;