muc_mam_markers: store chat markers in the Group Chat archives By enabling this module, messages containing XEP-0333 Chat Markers addressed to a Group Chat, will get stored in the archive.

This module blocks configured queries that change server state.

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

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