This is mostly motivated by the tchap use case, where usernames are
automatically generated from the user's email address (in a way that
allows figuring out the email address from the username). Therefore,
it's an issue if we respond to requests on `/register` and
`/register/available` with `M_USER_IN_USE`, because it can potentially
leak email addresses (which include the user's real name and place of
work).

If this config flag is turned on, Synapse ignores any username supplied
by the client on both endpoints, and instead generates a username
automatically for the user. In the future, there will be a module
callback that allows modules to decide which username to give the user
(by e.g. deriving it from an email address), so usernames generated with
this flag turned on won't be limited to integers.