REST: Handle unset series

REST: Handle unset series

This was introduced in the recent "convert series from N:M to 1:N"
series. We take the opportunity to make the 'create_patch' and
'create_cover' utility methods a little smarter, in that series will
automatically be created for the patch/cover letter unless told not to.
This requires some related changes to other modules.

Signed-off-by: Stephen Finucane <stephen@that.guru>