Clean up SimpleComment interface

Removing some fields I don't need in the app to make it easier for SimpleComment to be implemented by other sections that don't have this information
Clean up Inbox & Conversations

Fixing inbox paging by making the page number an instance variable, fix prev/next page checking and getting.

Adding support for the avatar field of messages in a conversation and having the messages in conversations implement SimpleComment so we can re-use the CommentsAdapter in the app to display them.
Add support for managing conversations

The site lets you mark conversations as sticky, unread or delete them. Support for this is now added to the API library
The app does need to set the body for request comments.
Merge branch 'feature-better-inbox' into develop