Now admins can choose a title for their About and Privacy pages; now
editable through the instance page editor.

This adds `title` and `content_type` fields to the `appcontent` table,
requiring a migration by running `writefreely --migrate`

The content_type field specifies that items we're currently storing in
this table are all "page"s; queries for fetching these have been updated
to filter for this type. In the future, this field will be used to
indicate when an item is a stylesheet (ref T563) or other supported
type.

Ref T566

This adds a "Pages" section to the admin part of the site, and enables
admins to edit the pre-defined About and Privacy pages there, instead of
on the dashboard itself.

It also restructures how these pages get sent around in the backend and
lays the groundwork for dynamically adding static pages. The backend
changes were made with more customization in mind, such as an
instance-wide custom stylesheet (T563).

Ref T566

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