Support page-{slug}.php and page-{id}.php template files for pages

[aaroncampbell]

According to the ​documentation the order for page templates is 'pagename.php'->'page.php'->'index.php' but it doesn't work like that. Currently it just supports 'selected custom template'->'page.php'->'index.php'. I talked to beaulebens and westi in IRC and suggested that the new order works more like the tag templates and goes like this:

'custom template'->'page-{slug}.php'->'page-{id}.php'->'page.php'->'index.php'

[westi]

(In [12135]) Improve get_page_template() to search based on slug and id as well. Fixes #11055 props aaroncampbell.