Move mailsync.backends.imapaccount -> mailsync.backends.imap.account.

Summary:
I don't think we should have direct submodules of mailsync.backends that
aren't actual backends. This code is clearly code that belongs to the
IMAP backend, and thus belongs in that submodule.

Test Plan: ./runtests passes

Reviewers: kav-ya

Differential Revision: https://review.inboxapp.com/D52
More linting and clarity fixes.

Summary:
Cleanups.

Test Plan: ./runtests passes

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D52