Rework loading images from files (#7088)

This PR is a follow-up to #7084, where I noted that I wasn't satisfied
with using `SharedUri` to represent both URIs and paths on the local
filesystem:

> I'm still not entirely happy with this naming, as the file paths that
we can store in here are not _really_ URIs, as they are lacking a
protocol.
>
> I want to explore changing `SharedUri` / `SharedUrl` back to alway
storing a URL and treat local filepaths differently, as it seems we're
conflating two different concerns under the same umbrella, at the
moment.

`SharedUri` has now been reverted to just containing a `SharedString`
with a URI.

`ImageSource` now has a new `File` variant that is used to load an image
from a `PathBuf`.

Release Notes:

- N/A

crates/collab_ui/src/chat_panel.rs

@@ -714,7 +714,7 @@ fn format_timestamp(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use gpui::{HighlightStyle, SharedUri};
+    use gpui::HighlightStyle;
     use pretty_assertions::assert_eq;
     use rich_text::Highlight;
     use time::{Date, OffsetDateTime, Time, UtcOffset};
@@ -730,7 +730,7 @@ mod tests {
             timestamp: OffsetDateTime::now_utc(),
             sender: Arc::new(client::User {
                 github_login: "fgh".into(),
-                avatar_uri: SharedUri::network("avatar_fgh"),
+                avatar_uri: "avatar_fgh".into(),
                 id: 103,
             }),
             nonce: 5,