closes#5551
- adds new test fixture generator and tests for tag pagination
- changes how post_count is added to use a select subquery rather than a join
closes#5490
- use same event handling pattern as fetchAll
- add support for `fetching:collection` to post model
- add tests to check that url is fetched via findAll and findPage
closes#2896
- move default options / custom code into model functions
- move most of the filtering logic into base/utils.filtering (to be relocated)
- move the remainder of findPage back into base/index.js and remove from posts/users&tags
- move pagination-specific logic to a separate 'plugin' file
- pagination provides new fetchPage function, similar to fetchAll but handling pagination
- findPage model method uses fetchPage
- plugin is fully unit-tested and documented
refs #2896
- moves repeated code out of models
- creates a new file for unit-testable code (this should be moved in future)
- adds a default for `page` as that seems sensible
- adds 100% test coverage for the new file
