Reduce the bundle size of attachments preview.

[deepakjosp]

Part of https://github.com/bigbinary/neeto-editor/issues/1123

• We use the react-doc-viewer package for showing the previews for attachments. This contributes to a significant amount of bundle size.
• Verify that if the attachments feature is not enabled, the package is not loaded into the main bundle.
• Hand-pick the required renderers from react-doc-viewer instead of using the DocViewerRenderers to load all renderers.