Change icon of quote notification mails (#35701)

app/javascript/images/mailer-new/heading/README.md

@@ -1 +1,3 @@
 Images in this folder are based on [Tabler.io icons](https://tabler.io/icons).
+
+Seems to be 1.5 width icons scaled to 64×64px and centered above a blue square with round corners (24px).

app/views/notification_mailer/quote.html.haml

@@ -1,6 +1,6 @@
 = content_for :heading do
   = render 'application/mailer/heading',
-           image_url: frontend_asset_url('images/mailer-new/heading/boost.png'),
+           image_url: frontend_asset_url('images/mailer-new/heading/quote.png'),
            subtitle: t('notification_mailer.quote.body', name: @status.account.pretty_acct),
            title: t('notification_mailer.quote.title')
 %table.email-w-full{ cellspacing: 0, cellpadding: 0, border: 0, role: 'presentation' }