Add an in-room reminder to set up key recovery

This adds an in-room reminder above the message timeline to set up Secure
Message Recovery so that your keys will be backed up. If you try to ignore it,
an additional dialog is shown to confirm.

Fixes vector-im/riot-web#7783.

Signed-off-by: J. Ryan Stinnett <jryans@gmail.com>

res/themes/light/css/_base.scss

@@ -155,6 +155,8 @@ $imagebody-giflabel-border: rgba(0, 0, 0, 0.2);
 // unused?
 $progressbar-color: #000;
 
+$room-warning-bg-color: #fff8e3;
+
 // ***** Mixins! *****
 
 @define-mixin mx_DialogButton {
@@ -187,3 +189,11 @@ $progressbar-color: #000;
     font-size: 15px;
     padding: 0px 1.5em 0px 1.5em;
 }
+
+@define-mixin mx_DialogButton_secondary {
+    // flip colours for the secondary ones
+    font-weight: 600;
+    border: 1px solid $accent-color ! important;
+    color: $accent-color;
+    background-color: $accent-fg-color;
+}