Admin (please review)

Please review this PR. It makes (relatively close to) the minimum of changes to admin, minimum in the sense of influencing the rest of the files. Plus some other refactorings. The admin rename proposed here is a change comparing to the initial intention, which was to heavily rename things to explicit names:
* it has the advantages of keeping things familiar and with significantly fewer changes overall; simpler names where possible; it'll help addons updates, and those coming from SMF.
* it has the disadvantage to _not_ make names themselves consistent. Arguably also: file/class names are less natural to those who don't know SMF.
The admin rename is probably the least important bit in this PR; yet, I think it matters to stay stable. I'd intend it to remain stable after this admin PR. So please bring your objections: if you're not comfortable working with it, I can implement the "NameAdmin".controller alternative.
They're quite similar design-wise, except in this PR, it's less polished or strict.

Summary:
* naming pattern for the files .controllers, .template, .subs/.class, language.
* admin pattern: not changed. 'Manage' as prefix wherever necessary, and nothing (simplest name) where possible.
* some refactoring of former Manage.subs, to the subs file dedicated to the entity (i.e. News.subs keeps low-level functions used/needed first by ManageNews)
* a pass at refactoring to create the list_ methods (callbacks of createList()). These can be improved, to share data through globals in the method (not the .subs function), as long as such globals are needed at all.
* refactoring ScheduledTasks.
* some refactoring of paid subscriptions (minimal), and add authorize.net and 2co gateways (minimal).