Rename PostCSS files to .pcss (#9013)

* Rename PostCSS files to `.pcss`

* Make Stylelint happy

* Delint

* Rename new files too

* delint

* Fix bad comment placement

res/css/structures/_AutoHideScrollbar.pcss

@@ -0,0 +1,62 @@
+/*
+Copyright 2018 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// make any scrollbar grey and thin
+html {
+    scrollbar-color: $scrollbar-thumb-color transparent;
+}
+// scrollbar-width is not inherited (but -color is, why?!),
+// so declare it on every element
+* {
+    scrollbar-width: thin;
+}
+
+::-webkit-scrollbar {
+    width: 6px;
+    height: 6px;
+}
+
+::-webkit-scrollbar-thumb {
+    border-radius: 3px;
+    background-color: $scrollbar-thumb-color;
+}
+
+// make auto-hide scrollbars not transparent again on hover
+.mx_AutoHideScrollbar:hover {
+    scrollbar-color: $scrollbar-thumb-color transparent;
+
+    &::-webkit-scrollbar-thumb {
+        background-color: $scrollbar-thumb-color;
+    }
+}
+
+// make scrollbars transparent for autohide scrollbars
+.mx_AutoHideScrollbar {
+    overflow-x: hidden;
+    overflow-y: auto;
+    overflow-y: overlay;    // where supported
+    -ms-overflow-style: -ms-autohiding-scrollbar;
+
+    &::-webkit-scrollbar {
+        background-color: transparent;
+    }
+
+    &::-webkit-scrollbar-thumb {
+        background-color: transparent;
+    }
+
+    scrollbar-color: transparent transparent;
+}