mod_http_admin_api: Tweak token session to please module:may() module:may() checks for type == "c2s", but mod_tokenauth does not currently include that or most common session properties. Fixes a traceback resulting from a different code path where module:may() tries to index event.stanza, which does not exist for http events.

This module blocks configured queries that change server state.

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

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