CMS pages: add options to show when created and last edited and by whom, with editable defaults

[CrispinF]

Four options in Settings:
Show created by?
Show created date?
Show last modified by?
Show last modified date?
To be rendered below content.
There should be settable default values for these under Content Settings, as these tend to be used or not used according to some editorial policy. But this default could be overridden on particular pages.

[markidox]

I've created a pull request for this. It will need quite a bit of testing as it touches various parts of the Simple Content system.
The settings (dubbed 'Editorial Settings') can be set from the Simple Content settings within admin, this should set them across standard pages and templates.
The settings also appear on each page, if they haven't been altered for that page, they will show the default settings.
New pages should be created, both from templates and from 'standard'.
Both sets of pages should be edited and deleted.
The Editorial Settings should be set/unset at a settings and individual page level.