Firebase used to do it automatically (with `cleanUrls: true`), but it
stopped doing it unless the resulting URL corresponds to an existing
file (which is not always the case in angular.io; e.g. the resulting URL
might be matched by a new redirect rule).
This change in Firebase hosting behavior resulted in some URLs not being
correctly redirected (e.g. URLs to the archived v2 site, or `.html`
suffixed URLs from 3rd-party sites).
This commit fixes it, by configuring Firebase hosting to strip off the
`.html` extension and redirect (if no other redirect rule matched).
PR Close#25999
df5999a739