Add shortcode for buttondown signup form and add to SiB page

.hugo/content/events/small-is-beautiful/index.md

@@ -28,7 +28,7 @@ _(For more frequent, development-related updates, see [Aral’s blog](https://ar
 
 ### Next Stream
 
-The next scheduled stream is on June 17, 2021.
+The next scheduled stream is on June 17, 2021. You can [get monthly reminders for future live streams below](#get-live-stream-reminders)
 
 #### Can the Small Web be as easy (or easier) to use than the Big Web? (Spoiler: Yes.)
 
@@ -54,3 +54,6 @@ __Join in!__ _Have your webcam/microphone and headphones ready if you want to jo
 
 [Watch live](/small-is-beautiful).
 
+### Get live stream reminders
+
+{{< notify >}}

.hugo/layouts/shortcodes/notify.html

+<p>Every month we’ll send you a reminder for the next episode of Small is Beautiful, the Small Technology Foundation
+livestream, so you can watch the stream live.</p>
+
+<p>That’s all we’ll send you! No spam or any other nonsense, and we will never use your email address for anything else.</p>
+
+<form action="https://buttondown.email/api/emails/embed-subscribe/small-is-beautiful" method="post" target="popupwindow"
+  onsubmit="window.open('notify.small-tech.org', 'popupwindow')" class="embeddable-buttondown-form">
+  <label for="bd-email">Your email address:</label>
+  <input type="email" name="email" id="bd-email" />
+  <input type="hidden" value="1" name="embed" />
+  <input type="submit" value="Subscribe" />
+</form>
+
+<p>If you’d prefer, you can also <a href='https://buttondown.email/small-is-beautiful/rss'>subscribe to monthly reminders via RSS</a>.
\ No newline at end of file