Create mastodon embed shortcode (#788)

This shortcode can be used as follows:

```
{{<mastodon server="" user="" id="">}}
```

It defaults to mastodon.social server.


Closes #787

Co-authored-by: Emruz Hossain <hossainemruz@gmail.com>

layouts/shortcodes/mastodon.html

@@ -0,0 +1 @@
+<iframe src="https://{{.Get "server" | default "mastodon.social"}}/@{{.Get "user"}}/{{.Get "id"}}/embed" class="mastodon-embed" style="max-width: 100%; border: 0" width="400" allowfullscreen="allowfullscreen"></iframe><script src="https://{{.Get "server" | default "mastodon.social"}}/embed.js" async="async"></script>