Now that auth flows can show a server name like `example.com` which might
delegate the HS to some other server, it could be confusing to see text like
"Sign in to example.com", especially if `example.com` runs an identity service,
uses SSO, has its own account system, or other things like this.
To clarify that we mean Matrix accounts, all auth flows are updated to talk in
terms of "<verb> your Matrix account on <server>".
Fixes part of
https://github.com/vector-im/riot-web/issues/8763#issuecomment-464823909.
d220dd49ef