Choose user to whom to assign content, instead of creating new users

Pulling over to main wordpress_migrate project.

I don't think the behavior of this patch is quite what we want in the general case. It appears to force all incoming authors whose email addresses aren't already attached to Drupal accounts to be mapped to the specified account - it leaves no means to create accounts in those cases, which should be the default behavior.

Possible behaviors (keeping in mind most WordPress blogs will only have a single author):

1. Current - create any author account(s) which don't already exist. All imported pages/posts get mapped to a Drupal account with the same email address as the WordPress author.

2. What the TODO was suggesting - pick an existing Drupal account to be the author of all imported pages/posts.

3. Patch behavior - If an existing Drupal account with a matching email address exists, use that as the author, otherwise use the chosen account.

So, we need to present these three options in some clear understandable way...

Committed with a label adjustment ("Assign unowned content to" seemed unclear, changed to "Default content author:"). Thanks!