This adds any OAuth login buttons to the invite signup page, stores the invite code for the flow duration, and associates the new user with it once successfully registered.
It enables invite-only instances with OAuth-based registration.
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
