Deduplicates code around extension marker and code intelligence injection in phabricator, integrations & injected script entrypoints.
Also removes comments about extension marker injection being racy in Phabricator and integration entry points: <script> tags with the defer attribute are executed after the document has been parsed, but before DOMContentLoaded fires. Meanwhile, browser extension content scripts are set to run at document_end, after DOMContentLoaded has fired.
