New mustache-powered paging_bar

Split out into an improvement from MDL-48664 so 2.8 can get some love over there.

This improvement involves:

• moving paging_bar into its own file in lib/classes/output/paging_bar.php
  • adjusting every use of paging_bar to \core\output\paging_bar in order to account for the namespace change
• adding a new Mustache template as lib/paging_bar.mustache
• adding the ability to hint the paging bar with regards to its display size (a new \core\output\paging_bar::displaysize option, which can be 'mini', 'small', or 'large'

The structure that the mustache template expects is:

{

    "type": "Page",

    "links": [

    	  {

            "href": "http://moodle.com",

            "text": "<3 Moodle",

            "attrs": []

        }

    ],

    "first": {

        "href": "http://moodle.com",

        "text": "0",

        "attrs": []

    },

    "last": {

        "href": "http://moodle.com",

        "text": "120",

        "attrs": []

    }

}