[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>

bskyweb/.gitignore

@@ -9,6 +9,10 @@ static/js/*.js
 static/js/*.map
 static/js/*.js.LICENSE.txt
 templates/scripts.html
+templates/*-embed.html
+static/embed/*.html
+static/embed/assets/*.js
+static/embed/assets/*.css
 
 # Don't ignore this file
 !.gitignore