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.
If a default server name is set and the current HS URL is the default HS URL,
we'll display that name in the "your account" text on the registration form.
This can be a bit more user friendly, especially when the HS is delegated to
somewhere such as Modular, since you'll then see "example.com" instead of
"example.modular.im", which you have no direct relationship with as a user.
This is the key bit of https://github.com/vector-im/riot-web/issues/8763 for
registration.
