mod_bookmarks2: Add new module. This is the result of hacking during the Stockholm XMPP Sprint, for compatibility with older clients only doing Private XML XEP-0048. This module shouldn’t be loaded at the same time as mod_bookmarks, as both implement Private XML to achieve their conversion.

This module blocks configured queries that change server state.

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

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