File UI redesign

[edemaine]

Here's a proposed overhaul of the Add/Replace File + Reply/Attach buttons at the bottom of each message:

• Reply/Attach menu as is, and it becomes the only button in those locations.
• File menu for managing the one file associated with a message.
  • When there isn't already a file: File menu appears only when editing message.
    • Add File can go here, perhaps renamed to something like "Embed Single File"
      • This reduces the prominence of "Add File" by two levels: you have to be editing, then you have to click File / Add File
  • When there is already a file: File menu is always next to the attachment top (where it says size and file type)
    • Replace File can go here
      • (Do we still want it next to Reply/Attach too, for easier discoverability? I think less necessary now that File will always be visible in a message with a file.)
    • Extract File into Reply: make new reply and move the message there
    • Move File: select another message to move the file to (target message must have no file currently)
    • Copy File: select another message to embed the same file in (via a pointer to the same file object); this is useful when you want the same image in two different threads, for example.
      • Perhaps only for Superusers? Rare that users need this level of control... But also not a dangerous operation, so maybe fine.
    • Remove File, in red, but this still needs a confirmation dialog
  • When viewing history, and there is a file:
    • Restore File: re-use this old version in current version of message (if they're not the same)
    • Extract File: like above, but without changing message
    • Copy File: as above
    • Superdelete File: When replacing old big files; see File list by size #279
• Superdelete should no longer delete all files throughout history of a message, as they might now be embedded elsewhere