Issues/1327 reader endpoints

Fixes #1327  

This pull request does a few things. 
- Replaces the old hardcoded reader topics with a list that is fetched from the REST API
- Adds a ReaderTopic model, and accompanying local and remote services.
- Refactors *most* of the networking and model logic in ReaderPost into services. There is some comment related code that we'll clean up later.
- Removes the relationship between Account and ReaderPost in favor of relationships between Account and ReaderTopic, and ReaderTopic and ReaderPost. 
- Adds a non-exhaustive suite of tests for the reader services.

The changes also close these issues:
- Closes #1609 - The /read/liked results are sorted by the date a post is liked.
- Closes #1372 - Topics are sorted a-z.
- Closes #1049 - Existing posts that have been removed from the endpoint (for whatever reason) are removed from core data. 
- Closes #411 - ReaderPosts belong to a ReaderTopic, which belongs to an WPAccount.  If an account is removed any topics and posts cascade.

Incidental changes:
- Changes the way ReaderPost's generate summaries.  To be more consistent with the Android app we're using the excerpt returned from the REST API with a fallback to one that is constructed if the excerpt is missing. 
- The old incomplete code related to tags has been removed.
- The clean up logic has changed. Instead of deleting content older than 7 days, we keep up to 200 posts via load more.  Currently pull to refresh still clears all but the newest results but this logic will change in a later pull request to match the new approach in the Android app.