Increased importer perf by 50% and allow adding nested relations by foreign key

[kirrg001]

no issue

Speed improvement

• change behaviour from updating user references after the actual import to update the user reference before the actual import
  • updating user references after the import is way less case intense
  • that was the initial decision for updating the references afterwards (was easier to implement)
  • but that does not play well with adding nested relations by identifier (foreign key)
• the refactoring is required for multiple authors
  • if we e.g. store invalid author id's, we won't be able to add a belongs-to-many relation for multiple authors
  • bookshelf-relations is generic and always tries to find a matching target before attching a model
  • invalid user references in the database are in general not acceptable
  • we have to keep backwards compatibility in the importer
  • e.g. you import author_id, we have to detect that and insert the correct post_authors relation
  • but if we try to add posts_authors with an invalid author_id, bookshelf-relations will complain, because it ensures that adding nested relations requires a valid and existent target (it does not allow adding broken references to the database)

This change has a very good side affect

• 17mb takes on master ~1,5m
  • on this branch it takes ~42seconds 🙊
• 40mb times out on master
  • on this branch it takes 1,5m
• memory looks like the same

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Insert nested tags by foreign key

• because of the importer change, we can proof that adding nested relations by foreign key works
• tags runs already through bookshelf-relations
• replace logic for preparing nested tags
  • we added nested relations by name
  • this is not needed, because the matching tag was already imported
  • also: the notation post.tags=[{name: 'x'}] doesn't even make use of the target tag id
  • bookshelf-relations is smart and can handle that case, but this notation looks like you would like to add a tag
  • adding nested tags by name can also trouble, because name is not even unique (!)
• what we do now
  • we simply would like to add the relationship to the database
  • use same approach as base class
  • add posts_tags to target post model
  • update identifiers
  • insert relation by foreign key tag_id e.g. post.tags=[{tag_id: '...'}]
• bump bookshelf-relations to 0.1.10 (which supports attaching relations by foreign key)

---

• tested example files in the browser
• push side-fixes to master straight (!)

@cobbspur I've assigned you. To A: share knowledge that we refactored a piece of the importer and B: it would be great if you could do a second browser test and compare the result 👍 The importer still works like the same, it's just faster. You don't have to review the code.

[kirrg001]

@cobbspur Could you please reserve a little time to test my PR? Thanks 👍 No code review needed.

[kirrg001]

Was tested by @kevinansfield, @AileenCGN and @cobbspur.
Will self-merge.