Publish to spring

Displays a member's springs on the new post interface (one checkbox for each). The new post will be added to the selected springs.

The nasty:

- *all* springs are displayed at the bottom of the post box in one long line.
- the post is distributed as before and then followed by *n* POST-POOL requests. I'd much rather have the POST request allow for a list of spring ids. Doing this with separate requests causes more overhead and might result in POST-POOL requests arriving before the POST request has been processed by the receiver.