Made attachments serve via URL instead of embedding them in the JSON response

[Servus-Altissimi]

As discussed in Issue 126 Large attachments and raw message bytes were being base64 encoded directly into the GET /api/message/{id} JSON response. This isn't an issue for smaller files, but larger attachments caused the UI to freeze, as the browser had to deserialize it all.

Hence I made the following changes:

• Attachment content and raw message bytes are no longer serialized into the JSON response
• Added endpoints to serve the raw message on demand
• Attachment downloads now point to the existing /api/message/{id}/attachment/{index} endpoint
• Inline (cid:) images in HTML emails are rewritten to use the attachment endpoint
• Raw tab content is fetched only when opened
• Added a test that sends a 75 MiB attachment and verifies it

I haven't bumped cargo version.