[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

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

bskyembed/assets/arrowBottom_stroke2_corner0_rounded.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" fill="none"><path fill="#000" fill-rule="evenodd" d="M12 21a1 1 0 0 1-.707-.293l-6-6a1 1 0 1 1 1.414-1.414L11 17.586V4a1 1 0 1 1 2 0v13.586l4.293-4.293a1 1 0 0 1 1.414 1.414l-6 6A1 1 0 0 1 12 21Z" clip-rule="evenodd"/></svg>

bskyembed/assets/bubble_filled_stroke2_corner2_rounded.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" fill="none"><path fill="rgb(10,122,255)" d="M19.002 3a3 3 0 0 1 3 3v10a3 3 0 0 1-3 3H12.28l-4.762 2.858A1 1 0 0 1 6.002 21v-2h-1a3 3 0 0 1-3-3V6a3 3 0 0 1 3-3h14Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24"><path fill="rgb(10,122,255)" d="M19.002 3a3 3 0 0 1 3 3v10a3 3 0 0 1-3 3H12.28l-4.762 2.858A1 1 0 0 1 6.002 21v-2h-1a3 3 0 0 1-3-3V6a3 3 0 0 1 3-3h14Z"/></svg>