Correct margins for heading actions (#12797)

This commit changes margins of the page heading header, actions in
order to fix displaying with low screen size and long enough header.

It is working by giving heading and action buttons margin from top
and then negating it in parent element. Whenever flex item wrap, the
"negated" margin will be applied again, providing us nice space
between header and action buttons.

This commit also adds a margin to header, so it does not clamp with
the heading actions and they wrap a little earlier (15px ahead). As
well as the left margin is not anymore applied to the first action.

app/javascript/styles/mastodon/admin.scss

@@ -186,17 +186,22 @@ $content-width: 840px;
 
       padding-bottom: 40px;
       border-bottom: 1px solid lighten($ui-base-color, 8%);
-      margin-bottom: 40px;
+
+      margin: -15px -15px 40px 0;
 
       flex-wrap: wrap;
       align-items: center;
-
       justify-content: space-between;
 
+      & > * {
+        margin-top: 15px;
+        margin-right: 15px;
+      }
+
       &-actions {
         display: inline-flex;
 
-        & > * {
+        & > :not(:first-child) {
           margin-left: 5px;
         }
       }