Make cross-signing dialog clearer and more context-aware

- Don't show loading spinners while waiting for user action
- When checking if there are other devices we can verify against, only
  look for devices that are actually cross-signed.
- Adjust displayed options depending on whether other devices and/or
  recovery keys exist, and add an option to reset cross-signing keys
  if necessary.
- Various minor clarifying adjustments to UI styling/text

Signed-off-by: Faye Duxovni <fayed@element.io>

res/css/structures/auth/_CompleteSecurity.scss

@@ -33,6 +33,19 @@ limitations under the License.
     margin: 0 auto;
 }
 
+.mx_CompleteSecurity_skip {
+    mask: url('$(res)/img/feather-customised/cancel.svg');
+    mask-repeat: no-repeat;
+    mask-position: center;
+    mask-size: cover;
+    width: 18px;
+    height: 18px;
+    background-color: $dialog-close-fg-color;
+    cursor: pointer;
+    position: absolute;
+    right: 24px;
+}
+
 .mx_CompleteSecurity_body {
     font-size: $font-15px;
 }

res/css/structures/auth/_SetupEncryptionBody.scss

@@ -0,0 +1,24 @@
+/*
+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_SetupEncryptionBody_reset {
+    color: $light-fg-color;
+    margin-top: $font-14px;
+
+    a.mx_SetupEncryptionBody_reset_link:is(:link, :hover, :visited) {
+        color: $warning-color;
+    }
+}