[sync/missing folders] Create folders on the backend when not present. Required for hovermail.

Summary:
Providers don't always provide all of the folders that we are
expecting. One such provider is hovermail. In order to support such providers,
we need to create these folders on-demand. This may not be the best place to do
so, however I couldn't think of a better place (perhaps initial sync?).

Fixes T446

Test Plan: sync hovermail account, perform self-send test.

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T446

Differential Revision: https://review.inboxapp.com/D506