Issue 227 login backout

This new version of the pull request remedies the problems @devgeeks found in #371:

1. Prior versions of the account model used .isLoggedIn to track login status, so that restored remembered instances would not be properly recognized when restored at app start by the new version of the account model. I added something AccountModel.initialize() to account for that, and remove the .isLoggedIn setting, in any case.
2. I've inhibited the posting of the error dialog in cases where the account login process was deliberately interrupted.

Also, this PR is directed to the master branch, rather than our now integrated master-ios-integration-branch.