Support editing About and Privacy pages from Admin panel

This allows admin to edit these pages from the web, using Markdown. It
also dynamically loads information on those pages now, and makes loading
pages templates a little easier to find in the code / more explicit.

It requires this new schema change:

CREATE TABLE IF NOT EXISTS appcontent (

`id` varchar(36) NOT NULL,
`content` mediumtext CHARACTER SET utf8 COLLATE utf8_bin NOT NULL,
`updated` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1;

This closes T533