Don't advertise in-band registration for read-only user providers

In-Band Registration (specified in https://xmpp.org/extensions/xep-0077.html ) is a mechanism that allows end-users to create new accounts.

Openfire allows this feature to be enabled or disabled using the Admin Console.

In-Band registration is very unlikely to be successful, if the UserProvider that's in use is read-only (which disallows creation, deletion or modification of users).

Openfire should not advertise the in-band registration feature, when the configured UserProvider is read-only.