Feature/1795 reader render attachments

Fix #1795 - There were two primary goals for this PR:

1. Simplify ReaderPostDetailFragment by moving the webView rendering logic to its own class
2. Provide device-specific heights and widths to IMG tags to (a) avoid having the content reflow as images are loaded, and (b) avoid downloading images larger than necessary

As an added bonus, these changes make it possible to display post content in any WebView, not just inside ReaderPostDetailFragment.