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>

src/async-components/views/dialogs/keybackup/CreateKeyBackupDialog.js

@@ -251,7 +251,7 @@ export default React.createClass({
             />
 
             <p>{_t(
-                "If you don't want encrypted message history to be availble on other devices, "+
+                "If you don't want encrypted message history to be available on other devices, "+
                 "<button>opt out</button>.",
                 {},
                 {