[mms] Linked attachments are no longer a user-configurable action.

[mms] Attachments can now be configured to be linked only if over a configurable size threshold.
Determine attachment limit on server side, not browser side
Attachment size limitations need to be checked at attachment time to produce the correct results
Hint to PHP server about max POST file size in dynamic view
IMP_Compose_LinkedAttachment::create() -> IMP_Compose#createLinkedAtc()
For pasted images, transfer to linked attachment sending if it exceeds the threshold