Don't show site config page in /-/admin/ activity view

It's confusing and dangerous.
Add test-embedded-site-manually E2E tests, and waitUntilBrowserClosed().
Admin dashboard: Use as topic URL any embedding page's URL.
Hide blog and forum related buttons for embedded comments discussions.
Really hide _site.conf

Don't know why it appears with 50% probability, but I'll rewrite the
admin dashboard in AngularDart anyway.
Fix E2E test: Add missing embedding pages.
Fix broken E2E test: tweak CSS selector.
Fix E2E test bug: Wait until login completed.
Somewhat fix broken E2E test and add comments.