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/message_editor.rs

@@ -365,7 +365,7 @@ impl Render for MessageEditor {
 mod tests {
     use super::*;
     use client::{Client, User, UserStore};
-    use gpui::{SharedUri, TestAppContext};
+    use gpui::TestAppContext;
     use language::{Language, LanguageConfig};
     use rpc::proto;
     use settings::SettingsStore;
@@ -392,7 +392,7 @@ mod tests {
                         user: Arc::new(User {
                             github_login: "a-b".into(),
                             id: 101,
-                            avatar_uri: SharedUri::network("avatar_a-b"),
+                            avatar_uri: "avatar_a-b".into(),
                         }),
                         kind: proto::channel_member::Kind::Member,
                         role: proto::ChannelRole::Member,
@@ -401,7 +401,7 @@ mod tests {
                         user: Arc::new(User {
                             github_login: "C_D".into(),
                             id: 102,
-                            avatar_uri: SharedUri::network("avatar_C_D"),
+                            avatar_uri: "avatar_C_D".into(),
                         }),
                         kind: proto::channel_member::Kind::Member,
                         role: proto::ChannelRole::Member,