Add options related to email attachments

Add options in the Mail archive administration page:

• "Store email attachments", checkbox, checked by default
• "Allow attachments in replies", checkbox, checked by default
• "Max attachment size", to allow only attachments under specified size
• "Authorized attachments", to specify a list of extensions (mime types ?) authorized for attachments
  The second one has meaning only if reply feature is implemented.
  Those options would allow admin to authorize or not storing attachments in created wiki pages, and to allow adding attachments when replying.
  Open questions:
• not sure if there should not be only a unique option for both
• should it be differentiated per server ?
• if "store" is unchecked, what do we do ? (reject emails containing attachments completely, add attachments names in loaded email, ...)