Support adding a caption to media uploads. (#3531)

* Add a composer to the MediaUploadPreviewScreen.

And send it's content to the media upload's caption.

* Use the new compound SendButton (updating relative padding in the toolbar).

* Update snapshots.

* Add unit tests for sending a caption.

ElementX/Sources/Screens/MediaUploadPreviewScreen/MediaUploadPreviewScreenModels.swift

@@ -15,6 +15,13 @@ struct MediaUploadPreviewScreenViewState: BindableState {
     let url: URL
     let title: String?
     var shouldDisableInteraction = false
+    
+    var bindings = MediaUploadPreviewScreenBindings()
+}
+
+struct MediaUploadPreviewScreenBindings: BindableState {
+    var caption = NSAttributedString()
+    var presendCallback: (() -> Void)?
 }
 
 enum MediaUploadPreviewScreenViewAction {