Add read-only support to Reddit via RSS feeds

**Is your feature request related to a problem? Please describe.**
The Fediverse still does not have enough content (compared to Reddit) to make it appealing for the majority of people, so lots of people still keep the habit of browsing Reddit, even if only for lurking.

There are a good number of Lemmy instances that are dedicated to mirroring Reddit content, but due to how federation works, this can lead *all* instances to dealing with copyright issues.

One way to solve both of these issues would be by having Voyager able to *browse* the content from Reddit feeds, and this should be possible to do by using the RSS feeds provided by Reddit.

**Describe the solution you'd like**
An additional tab next to "Local/All" which would list posts and discussions from Reddit, fetched via RSS. All the functionality to display posts and comments would work like they do for Lemmy posts, but any other functionality that requires interaction with the API (votes, replies, moderation actions, etc) would be disabled.

**Describe alternatives you've considered**
Using Reddit Feeds as source of content is something that I am already doing on https://fediverser.network, but that site is only meant to help people to migrate from Reddit to Lemmy, so it does not work well as a Lemmy client.