move mail leads to copy -> expunge

[jrse]

rbox plugin currently handles a move command with copy and expunge. But mdbox is able to detect a move commands. (via ctx->moving).

• rbox move should result in copy -> expunge in case move is done between users.
• rbox move from folder to folder -> move should be done differently by modifying index entry.

[pburow]

There is no way to move messages between users. So the first point (rbox move should result in copy -> expunge in case move is done between users) isn't implemented.