Configure one app as the default

A user should be able to configure one app as the default app for a FreedomBox. For example, if MediaWiki is configured as the default, MediaWiki will become the landing page for the FreedomBox instead of Plinth. This should be a new configuration option with /plinth as the default value but can be changed to some other path like /mediawiki. This feature can be the starting point for the use case where a user wants to install an instance of one big application like MediaWiki, GitLab or Diaspora and uses FreedomBox to install and maintain it.

This feature can also help people host simple websites on their FreedomBoxes. For example, by dumping static content into /var/www/html/website and configuring /website as the default landing page.