add a random tag to our default XMPP resource value

if the user is still using our default "ChatSecure" resource, then
we should append it once with a random alphanum value so that there
are no issue with more than one device with the same resource is logged in
to same server at the same time
see: https://pthree.org/2007/07/13/why-xmpp-part-2-saving-your-resources/