Improved preview

Some minor fixes and improvements of the file browser and preview functionality:

• the FilePreview component is moved to a separate file
• code is refactored by introducing viewer objects, one for each file type we support. The viewer object stores constant information, like matching file extensions, maxSizeAutoPreview etc, plus a render function.
• a viewer is introduced for json files (the same viewer used elsewhere).
• txt, yaml and yml files are automatically previewed with the text viewer
• error handling is added. If the viewer fails, we show a message and offers the user to view the file using the text viewer instead.
• optimizing the vertical file browser api calls, using the include_entry_info option instead of making a costly second api call.
• Highlighting the current entry mainfile