Hook events into Markdown render using ref.

Temporary change: Prevent TextFile from loading large content.
Fixes #1: File preview failing with JSON files.
Add a way to open text files in new tab.

- attachment scaling now works for all forms of attachments
- switched to grid based action bar
- added onLoad class so image proportions are "trimmed" once the image has loaded to avoid moving the view around before loading
  - this may be possible to remove at some point