Store session per full JID

Otherwise we run into a bug where two tabs with Converse.js share the
same XEP-0198 SM-ID, causing both to go into a reconnection-loop as
the XMPP server switches XEP-0198 sessions between them.

This bug is due to a distinction in how sessionStorage behaves when you
open the existing site in a new tab (e.g. middle-click or
`target="_blank"), as opposed to creating a new tab
and then opening the site in that tab.

In the latter case, the newly created sessionStorage object is empty.
In the former, the contents of sessionStorage of the current page is
copied over to the new page!

src/headless/converse-core.js

@@ -506,8 +506,10 @@ _converse.initConnection = function () {
 
 
 async function initUserSession (jid) {
-    const bare_jid = Strophe.getBareJidFromJid(jid);
-    const id = `converse.session-${bare_jid}`;
+    // XXX: Important to use full JID, otherwise we run into a bug where two
+    // tabs with share the same XEP-0198 SM-ID, causing them to go into a
+    // reconnection-loop.
+    const id = `converse.session-${jid}`;
     if (!_converse.session || _converse.session.get('id') !== id) {
         _converse.session = new Backbone.Model({id});
         _converse.session.browserStorage = new BrowserStorage.session(id);