Privately follow RSS feeds and make share-/quote-able

### Problem I have

I would love to be able to follow RSS feeds and the feed appearing mixed into my normal timeline.

### Solution I'd like

Obviously I would not be able to leave a like or comment. But I think it would be cool to show a special type of post for RSS feed "messages", and they could have the feature of "Quote", which would merely be creating a template with the header and link to the content and some information that this is a "RSS share" (or similar) that can then be annotated by the user and/or shared directly.

### Alternatives considered

_No response_

### Other

_No response_

The issue names no files, tests, or entry points, so begin by mapping the timeline and posting flows in this Mastodon web client. Define how users add and follow RSS feeds, how feed messages appear in the normal timeline, and how quote or share actions should work; done means the behavior is implemented and verified for those flows.

Written by the indexing model from the issue text.