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/notifications/stories/collab_notification.rs

@@ -1,4 +1,4 @@
-use gpui::{prelude::*, SharedUri};
+use gpui::prelude::*;
 use story::{StoryContainer, StoryItem, StorySection};
 use ui::prelude::*;
 
@@ -19,7 +19,7 @@ impl Render for CollabNotificationStory {
                 "Incoming Call Notification",
                 window_container(400., 72.).child(
                     CollabNotification::new(
-                        SharedUri::network("https://avatars.githubusercontent.com/u/1486634?v=4"),
+                        "https://avatars.githubusercontent.com/u/1486634?v=4",
                         Button::new("accept", "Accept"),
                         Button::new("decline", "Decline"),
                     )
@@ -36,7 +36,7 @@ impl Render for CollabNotificationStory {
                 "Project Shared Notification",
                 window_container(400., 72.).child(
                     CollabNotification::new(
-                        SharedUri::network("https://avatars.githubusercontent.com/u/1714999?v=4"),
+                        "https://avatars.githubusercontent.com/u/1714999?v=4",
                         Button::new("open", "Open"),
                         Button::new("dismiss", "Dismiss"),
                     )