Describe the bug
It is possible to send messages with an empty subject line.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://runbox.com/app/compose?new=true
2. Enter a recipient address and click to send the message
3. No warning is shown that the subject field is empty.

Expected behavior
A warning should be shown requesting confirmation that that message should be sent without a subject line.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: macOS
• Browser: Firefox