UX: main-outlet needs dynamic bottom padding when composer open (#119)

When the composer is open the `#main-outlet`'s bottom padding needs to
match the composer height, so the content behind the composer can be
scrolled into view.

Reported here:
https://meta.discourse.org/t/help-us-test-horizon-our-newest-theme/360484/26?u=awesomerobot

scss/main.scss

@@ -63,8 +63,11 @@ body:not(.has-full-page-chat) {
     }
     #main-outlet {
       width: 100%;
-      padding-bottom: var(--spacing-block-l);
       max-width: unset;
+      padding-bottom: var(--spacing-block-l);
+      html.composer-open & {
+        padding-bottom: var(--composer-height);
+      }
       > *:not(.experimental-screen, .activate-account) {
         @include breakpoint(medium, $rule: min-width) {
           box-sizing: border-box;