Only fill entry model with id if it already has one, to prevent postgres error

[helloiamlukas]

fixes #67

[edalzell]

@helloiamlukas can you please resolve the merge issues?

[what-the-diff]

• The id attribute is now optional when creating a new entry.
• A bug was fixed where the Entry model would not be found if it did not have an ID set yet, but had other attributes that could be used to find or create the record (e.g., origin_id).

[jasonvarga]

FYI currently this reverts the fix from #19. If you were to import entries, they'd lose their existing IDs.

I'll update this PR appropriately.