Add option to set file sharing component for blog posts

[ghost]

I brought this up a while back in the Movim MUC, but I figured I should add this here.

Since most XMPP servers are set to only retain messages (and by extension, shared files) for a week or month, Movim doesn't work all that well for art blogs or any other blogs that are centered around sharing files. An option that could fix that is allow the administrator to set a custom file sharing component.

For example: Say a prosody server has the typical file sharing component at share.example.com that has whatever retention the admin sets it to that fits well for chatrooms, but also has another files sharing component at movim-share.example.com with a much longer retention period and has disco_hidden = true set so XMPP clients don't use it by default.

With an administrator accessible option in Movim, the admin could set movim-share.example.com to be used for attachments in blog posts