New API for selecting a task; related refactoring and minor fm

* New API module GettingStartedGetPages for getting one or more pages for a given task
* Used by special page
* New ArticleFilter for secondary criterion (other than Redis set) that can allow or veto an article
** Broken out from the special page.
* Some renaming for clarity
* Small README fix about srand.

Bug: 55598
Bug: 55773
Change-Id: Icaf84e75d95bb06d108137221aa13e99003355e1
Merge "New API for selecting a task; related refactoring and minor fm"