nixos/prosody: add MUC extraConfig, fix extraConfig order

Add extraConfig option for the muc submodule.

Also move the global extraConfig before all components and
virtualhosts, because the manual states:

    The configuration is divided into two parts. The first part is known as
    the "global" section. All settings here apply to the whole server, and
    are the default for all virtual hosts.

    The second half of the file is a series of VirtualHost and Component
    definitions. Settings under each VirtualHost or Component line apply
    only to that host.

Before, if at least one muc was defined, or uploadHttp enabled, the
global extraConfig would end up after "muc" or "http_upload" component
making it apply to that component only and not globally.