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;
 }