Adopt Compound Colours (#1086)

* Delete deprecated font tokens.

* Migrate from primaryContent to textPrimary/iconPrimary.

* Migrate from links to textLinkExternal.

* Migrate from secondaryContent to textSecondary/iconSecondary.

* Migrate from accent to textActionPrimary.

- Some uses changed to textPrimary or iconPrimary as necessary.
- Some manual tints removed now that that Sentry is fixed.

* Migrate alert to textCriticalPrimary/iconCriticalPrimary.

* Migrate from brand to textActionAccent/iconAccentTertiary.

* Migrate from background to bgCanvasDefault or text/iconOnSolidPrimary

* Migrate system to bgSubtleSecondary.

* Remove ElementUIColors and obsolete migrated colours.

* Migrate tertiaryContent/quaternaryContent/quinaryContent to Compound.

* Migrate bubblesYou/bubblesNotYou to use Compound.

* Update Compound.

ElementX/Sources/Other/SwiftUI/Views/PlaceholderAvatarImage.swift

@@ -49,11 +49,11 @@ struct PlaceholderAvatarImage: View {
 
     private var bgColor: Color {
         if redactionReasons == .placeholder {
-            return .element.systemGray4
+            return Color(.systemGray4) // matches the default text redaction
         }
 
         guard let contentID else {
-            return .element.accent
+            return .compound.iconPrimary
         }
 
         return .element.avatarBackground(for: contentID)