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 lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_whitelist",

}
s2s_whitelist = {
    "example.org",
}
```