Make sure flow entries are created with correct content
BoardContent objects will store a string of json that includes
the workflow id. However, when creating the board, the workflow
has not yet been stored (and the workflow can't be stored until
the page has been stored)
SubmissionHandler used to initialize BoardContent with null if
the workflow was new, which led to revision with invalid content
(the workflow id was not in there) so it wasn't possible to,
for example, delete the page.
Let's just create the board with the workflow id of the board
we're about to commit a couple of lines later. In the unlikely
event that goes wrong (validation has already occured at this
point), we can remove the page again, thus ensuring a page
can't exist without a workfow and vice versa.