Delete topics when account changes.
Updates ReaderPost entity making a topic relationship required.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1778-delete-topics
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1778-delete-topics
Add tests for reader cleanup.

Fixes broken tests for accounts and concurrency.
Removes .h file from other test.
Fix for showing posts opened via a url scheme in the reader.

Misc formatting tweaks.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1778-delete-topics
Clean up one-off posts with no topic, like those loaded via URL scheme

Minor code style tweak
Records error that was previously ignored.
Handle the current topic being deleted on the server.

Classes do not keep their only references to the current topic
Properly log errors fetching the current topic by ID.
Handle unsubscribed topics that are also recommended.
Refactor for clarity
Add missing negation to comparison check.
Correctly set, get, and delete current topic.

Clear currentTopic when deleting topic
When retriving currentTopic, let first error check fall through.
Ensure currenTopic's objectID is not temporary.
Save context via ContextManager
Add support for obtaining object's permanent ID to ContextManager
Encapsulate state for backfill calls.
Adds a check for nil.
Merge pull request #1803 from wordpress-mobile/issues/1778-delete-topics

Issues/1778 delete topics
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into feature/pod-editor

* 'develop' of github.com:wordpress-mobile/WordPress-iOS:
  Adds a check for nil.
  Encapsulate state for backfill calls.
  Add support for obtaining object's permanent ID to ContextManager
  Correctly set, get, and delete current topic.
  Add missing negation to comparison check.
  Refactor for clarity
  Handle the current topic being deleted on the server.
  Clean up one-off posts with no topic, like those loaded via URL scheme
  Fix for showing posts opened via a url scheme in the reader.
  Add tests for reader cleanup.
  Updates ReaderPost entity making a topic relationship required.
  Delete topics when account changes.