Potentially fix T197, T194.

Summary:
T197 was due to yesterday's "fix" to T194.
Therefore revert it (i.e. remove the db_session.add(folder) in create_db_objects();
if no new_uids were added, there is no db_commit() before the save_folder_names() which
will try to create the same folder raising the error in T197).

This didn't really fix T194 anyway so it's not a huge loss.

So, take two at fixing T194:
Folder.create() adds the created Folder to the session;
there is some weirdness about association proxy + backref
(See: https://groups.google.com/forum/#!topic/sqlalchemy/6RV6ay3rwIs).
In particular, I'm not sure how autoflush behaves in this case when
the folder the FolderItem is instantiated with may not have been
explicitly added to the session.

As before, not certain this is a guaranteed fix but:
1. I'd like to test it against mg's failing accounts on Gunks.
2. I won't mark anything as resolved till we see it is.
3. It may work.

Test Plan: Sync on inboxapptest runs.

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T194

Differential Revision: https://review.inboxapp.com/D264