Add well-known url for changing passwords.

As per https://news.ycombinator.com/item?id=18618193, someone can put a .well-known/change-password redirect to the page where you can change your passwords. This helps password managers to link you the correct site. More explanation here: https://github.com/WICG/change-password-url/blob/gh-pages/explainer.md

Actual spec: https://wicg.github.io/change-password-url/index.html

This probably touches different teams I imagine, because the solution could be different for gitlab.com and self-hosted.

Example:

$ curl -I https://www.icloud.com/.well-known/change-password
HTTP/1.1 301 Moved Permanently
Server: AppleHttpServer/2f080fc0
Content-Type: text/html; charset=iso-8859-1
Location: https://appleid.apple.com/account/manage
Strict-Transport-Security: max-age=31536000; includeSubDomains
Access-Control-Expose-Headers: X-Apple-Request-UUID
Access-Control-Expose-Headers: Via
Date: Thu, 06 Dec 2018 15:01:07 GMT
Connection: keep-alive