ui: Improve UX around clearing cache

Testing
-------

PASS

Device: iPhone 14 Pro simulator
iOS: Tested on iOS 17.0 and 16.4
Steps:

1. Go to appearance settings
2. Enable animations. Shows confirmation dialog. PASS
3. Click cancel. Setting is toggled back. PASS
4. Enable animations again. This time click "OK". Setting stays at what was set, and cache is visibly cleared. PASS
5. Restart app. Changes are persistent. PASS
6. Disable animations. Dialog appears like before. PASS
7. Cancel. Toggles back as expected. PASS
8. Disable animations again. This time click "OK". Cache is cleared. PASS
7. Restart app. Changes are persistent. PASS
9. Click on "clear cache". Confirmation dialog appears. PASS
10. Cancel action. We do not see cache being cleared. PASS
11. Click on "clear cache" and click "OK" this time.
12. We can see the cache being visibly cleared. It shows a loading spinner and "clearing cache", and then we see a checkmark icon with a "cache cleared" indicator. We cannot click the button again for now. PASS
13. Go to home view, scroll through some views, then come back to the setting. Clear cache button is visible again.

Closes: https://github.com/damus-io/damus/issues/1301
Changelog-Changed: Improve UX around clearing cache
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/ConfigView.swift

@@ -175,8 +175,22 @@ func handle_string_amount(new_value: String) -> Int? {
     return amt
 }
 
-func clear_kingfisher_cache() -> Void {
+func clear_kingfisher_cache(completion: (() -> Void)? = nil) {
     KingfisherManager.shared.cache.clearMemoryCache()
-    KingfisherManager.shared.cache.clearDiskCache()
-    KingfisherManager.shared.cache.cleanExpiredDiskCache()
+    
+    let group = DispatchGroup()
+
+    group.enter()
+    KingfisherManager.shared.cache.clearDiskCache {
+        group.leave()
+    }
+
+    group.enter()
+    KingfisherManager.shared.cache.cleanExpiredDiskCache {
+        group.leave()
+    }
+
+    group.notify(queue: .main) {
+        completion?()
+    }
 }