DEV: Put full width styles into their own file (#132)

Followup f1d5b69293

We want to make it super clear that this is to be removed when we merge
full width into core, keeping it in desktop.scss together with other
styles will make that harder.

scss/desktop-horizon-fixes.scss

@@ -0,0 +1,65 @@
+// Fixing bulk select (only needed for desktop)
+
+.bulk-select-enabled {
+  .topic-list-header .topic-list-data.default {
+    position: sticky;
+    top: 10em;
+  }
+  .topic-author-avatar-data {
+    display: none;
+  }
+
+  .bulk-select.topic-list-data {
+    grid-area: bulk-select;
+    margin-left: -0.5em;
+
+    @media screen and (max-width: 576px) {
+      margin-top: 0;
+      label {
+        padding-block: 0.345em;
+      }
+    }
+
+    input {
+      transform: scale(1.5);
+    }
+  }
+}
+
+.topic-list-header {
+  tr {
+    border: none;
+  }
+
+  .topic-list-data {
+    padding: 0;
+
+    &:not(.default) {
+      display: none;
+    }
+
+    &.default {
+      position: absolute;
+      right: 0;
+      .bulk-select,
+      span:not(.bulk-select-topics, .d-button-label) {
+        display: none;
+      }
+    }
+  }
+
+  .bulk-select-topics {
+    position: absolute;
+    right: -1em;
+    @media screen and (max-width: 1048px) {
+      right: 0;
+    }
+
+    background: var(--secondary);
+    border-radius: 0 0 0 var(--d-border-radius);
+    padding: 1em;
+    button {
+      white-space: nowrap;
+    }
+  }
+}