[Privacy Issue] Add option to disable the automatic loading of images in Direct/Private Messages

**Is your feature request related to a problem? Please describe.**
Users can design malicious private messages that automatically harvest a users IP address when opened.
This is done by hosting the image on a service that records IPs or on a server that they control. The image is then automatically loaded by Voyager when opened. A new image is configured for each individual they send it to, so that they know only that individual will load it.

**Describe the solution you'd like**
A Toggle that disables the automatic loading of images in private/direct messages. I also feel like it should be on by default.

**Describe alternatives you've considered**
I think this is the only way to improve privacy in this situation.

**Additional context**
A [discussion](https://github.com/LemmyNet/lemmy-ui/discussions/1948) on the Lemmy GitHub talks about how direct messages in Lemmy are problematic.