(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.
(Bug1867) Fix readfilters module ordering

Fix the readfilters module ordering for Blanket and EasyRead.
(Bug 1867) Review code clean up

This patch:

* Standardizes the module name around subscription filter; that's what they're called
  in other areas of the site, so we should be consistent.
* Standardized the SubscriptionFilter class to be consistent in naming; it now has
  a string url var instead of a string link, because the Link class seemed overkill
* Abstracts away the id of the Subscription filter; we can add it in if we need it later
* Improves the logic of adding a filter to the list of filters displayed
(Bug 1867) Add a manage link to the filter module

Implements the same way other management links have been implemented.
(Bug1867) Improve filter module print checks

Now the module checks to see if it should print:

* It shouldn't print for communities or for syncdication journals
* It shouldn't print if there are no filters for it to print
(Bug1867) Filter module shouldn't show for comms

Makes it so the filter module doesn't show up in the
theme customization wizard for communities, as they
cannot use it.
Merge pull request #272 from foxfirefey/Bug1867/readfiltermodule

(Bug 1867) Add reading filter module to core2