[Embeds] Embed subdomain landing page (#3501)

* add build output to web build

* simplify post-build step by copying everything at once

* make script that converts placeholder -> iframe

* dynamically resize iframe based on inner content

Requires the iframe content to `postMessage` its height back up to the parent

* add lang to embed

* svg explicit height -> viewBox

* add build output to web build

* simplify post-build step by copying everything at once

* attempt to fix go embed issue

* rm changes to bskyweb

* remove another bskyweb change

* embed landing page

* Drop xl breakpoint, too far down

* Remove pointer enter behavior

* Avoid button width jump

* Escape HTML

---------

Co-authored-by: Dan Abramov <dan.abramov@gmail.com>

bskyembed/tsconfig.snippet.json

@@ -0,0 +1,10 @@
+
+{
+  "compilerOptions": {
+    "target": "ES5",
+    "lib": ["DOM", "DOM.Iterable", "ESNext"],
+    "strict": true,
+    "outDir": "dist"
+  },
+  "include": ["snippet"],
+}