Add-on for send-unsent-messages-without-attachments

[b18050]

@darktrojan An add-on which will add an option to send unsent messages without attachments . While travelling on a plane, I check my downloaded emails and may reply /send many emails . Some are brief , others have large attachments. When I connect to a mobile network with low data range. Thunderbird tries to deliver the emails in Outbox in chronological order. If the first on happens to be a low-priority email with large attachment,it may take a very long time or even timeout. Short high-priority emails later in the Outbox are blocked.