fix(plugins): fix getStubManuscriptObject

When populating blank values for the submission object, from fields specified in the form, we were failing to break the field name down into its parts. So we got a manuscript object like:

{
  id: 'abc1234blahblah',
  meta: { title: '' },
  submission: {
    'meta.title': '',
    'submission.firstAuthor': 'Joe Bloggs',
    'submission.label': '',
    ...
  },
  ...
}

We should have been discarding all fields whose name didn't start with 'submission.', and splitting off the 'submission.' when inserting into the submission object. With these fixed we get a manuscript object like this:

{
  id: 'abc1234blahblah',
  meta: { title: '' },
  submission: {
    firstAuthor: 'Joe Bloggs',
    label: '',
    ...
  },
  ...
}