Add dialpad to transfer dialog + various dialpad UI improvements (#6363)

Co-authored-by: Germain <germain@souquet.com>
Co-authored-by: Andrew Morgan <andrew@amorgan.xyz>
Co-authored-by: David Baker <dbkr@users.noreply.github.com>

res/css/views/elements/_DialPadBackspaceButton.scss

@@ -0,0 +1,40 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+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_DialPadBackspaceButton {
+    position: relative;
+    height: 28px;
+    width: 28px;
+
+    &::before {
+        /* force this element to appear on the DOM */
+        content: "";
+
+        background-color: #8D97A5;
+        width: inherit;
+        height: inherit;
+        top: 0px;
+        left: 0px;
+        position: absolute;
+        display: inline-block;
+        vertical-align: middle;
+
+        mask-image: url('$(res)/img/element-icons/call/delete.svg');
+        mask-position: 8px;
+        mask-size: 20px;
+        mask-repeat: no-repeat;
+    }
+}