(Bug 1867) Add reading filter module to core2

* adds in a SubscriptionFilter class to S2
* adds in a built-in function to S2, journal_subscription_filters, returning a list of
  SubscriptionFilters
* adds in appropriate module variables in core2
* adds a "print_module_readfilters()" S2 function to print the module; it uses the standard
  module list printing
* adds "property use module_readfilters_group;" to all propgroup modules sections in layouts

Things to double check:

* Test that private filters do not show to people other than the journal owner; I've tested this but it's very critical.