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/elements/_Validation.pcss

@@ -0,0 +1,69 @@
+/*
+Copyright 2019 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.
+*/
+
+.mx_Validation {
+    position: relative;
+}
+
+.mx_Validation_details {
+    padding-left: 20px;
+    margin: 0;
+}
+
+.mx_Validation_description + .mx_Validation_details {
+    margin: 1em 0 0;
+}
+
+.mx_Validation_detail {
+    position: relative;
+    font-weight: normal;
+    list-style: none;
+    margin-bottom: 0.5em;
+
+    &:last-child {
+        margin-bottom: 0;
+    }
+
+    &::before {
+        content: "";
+        position: absolute;
+        width: 14px;
+        height: 14px;
+        top: 0;
+        left: -18px;
+        mask-repeat: no-repeat;
+        mask-position: center;
+        mask-size: contain;
+    }
+
+    &.mx_Validation_valid {
+        color: $accent;
+
+        &::before {
+            mask-image: url('$(res)/img/feather-customised/check.svg');
+            background-color: $accent;
+        }
+    }
+
+    &.mx_Validation_invalid {
+        color: $alert;
+
+        &::before {
+            mask-image: url('$(res)/img/feather-customised/x.svg');
+            background-color: $alert;
+        }
+    }
+}