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/views/spaces/_SpaceBasicSettings.pcss

@@ -65,7 +65,7 @@ limitations under the License.
             display: inline-block;
             margin: auto 18px;
             color: $links;
-            font-size: $font-14px; // See _SpaceSettingsDialog.scss
+            font-size: $font-14px; // See _SpaceSettingsDialog.pcss
         }
 
         > .mx_SpaceBasicSettings_avatar_remove {