Move attachment-related options in dynamic compose screen to dropdown adjacent to attachment upload element
Make linking attachments a run-time decision when calling buildAndSendMessage()
[mms] Support link attachment feature in dynamic view.
[mms] Converted 'link_all_attachments' configuration option to the 'link_attach' preference.

Allows for more granular control of the display, and allows the default
value to be set while still allowing a user to unset it.