MDL-68828 theme_classic: fix for mobile

- setting a max width so the page does not overflow its container on
mobile
- fixing dashboard cards to be 100% wide on mobile

theme/classic/scss/classic/post.scss

@@ -112,6 +112,7 @@
     .blockcolumn,
     .region-main {
         flex: 0 0 100%;
+        max-width: 100%;
         padding: 0 1rem;
         margin-bottom: 1rem;
     }
@@ -130,11 +131,13 @@
     @include page_layout(20%);
 }
 
-.block_myoverview,
-.block_recentlyaccesseditems {
-    .dashboard-card-deck {
-        .dashboard-card {
-            width: calc(33.33% - #{$card-gutter});
+@include media-breakpoint-up(sm) {
+    .block_myoverview,
+    .block_recentlyaccesseditems {
+        .dashboard-card-deck {
+            .dashboard-card {
+                width: calc(33.33% - #{$card-gutter});
+            }
         }
     }
 }

theme/classic/style/moodle.css

@@ -18789,6 +18789,7 @@ span[data-flexitour="container"][x-placement="right"], span[data-flexitour="cont
   .blockcolumn,
   .region-main {
     flex: 0 0 100%;
+    max-width: 100%;
     padding: 0 1rem;
     margin-bottom: 1rem; } }
 
@@ -18972,9 +18973,10 @@ span[data-flexitour="container"][x-placement="right"], span[data-flexitour="cont
     padding: 0 1rem;
     max-width: 20%; } }
 
-.block_myoverview .dashboard-card-deck .dashboard-card,
-.block_recentlyaccesseditems .dashboard-card-deck .dashboard-card {
-  width: calc(33.33% - 0.5rem); }
+@media (min-width: 576px) {
+  .block_myoverview .dashboard-card-deck .dashboard-card,
+  .block_recentlyaccesseditems .dashboard-card-deck .dashboard-card {
+    width: calc(33.33% - 0.5rem); } }
 
 @media (min-width: 768px) {
   .blockcolumn .dashboard-card-deck {