mod_http_oauth2: Add a special "xmpp" scope that grants the users' default role This will be the first step towards defining a standard set of XMPP scopes. "xmpp" behaves as an alias for the user's default role, so that the client does not need to know about the various prosody:* roles.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```