[IMP] website_slides: improve not pretty thumbnail

In 9.0 image_medium and image_thumb route are used to display thumbnail
of website_slides document.

These in 9.0 will scale the image, but also crop it so it has a good
aspect ratio.

In saas-6 this croping feature is not present and these thumbnail will
be scaled in diverse ugly way.

To improve a little the situation, this commit use some CSS (currently
not supported only by internet explorer) which give a similar aspect as
9.0's got.

opw-658957

addons/website_slides/static/src/less/website_slides.less

@@ -11,6 +11,11 @@
     @navbar-default-bg: #f8f8f8;
 } .define-color-if-not-defined();
 
+.top_covered_image {
+    object-fit: cover;
+    object-position: top;
+}
+
 .oe_slides_panel_footer {
     background-color: @navbar-default-bg;
 }
@@ -44,6 +49,7 @@ section.oe_slides_promote_box {
 }
 
 img.oe_slides_channel_thumbnail {
+    .top_covered_image();
     height: 250px;
     width: 100%
 }
@@ -53,6 +59,7 @@ img.oe_slides_opacity {
 }
 
 img.oe_slides_promote_image {
+    .top_covered_image();
     max-height: 300px;
     width: 100%;
     overflow: hidden;
@@ -64,6 +71,7 @@ img.oe_slides_promote_image {
 }
 
 .oe_slides_apart_small {
+    .top_covered_image();
     height: 60px;
     width: 90px;
 }
@@ -92,6 +100,7 @@ img.oe_slides_promote_image {
 }
 
 .oe_slides_grid_thumbnail {
+    .top_covered_image();
     height: 200px;
     width: 100%;
     overflow: hidden;