iOS: show message sent/unread status (#293)

* light github image for dark mode

* show message received status, remove chevrons in chat list

* show unread message status

* add message send error mark

* refactor alerts to use AlertManager

* show alert message on tapping undelivered message, simplify text-only alerts

apps/ios/Shared/Assets.xcassets/github_light.imageset/Contents.json

@@ -0,0 +1,23 @@
+{
+  "images" : [
+    {
+      "filename" : "github_light_1x.png",
+      "idiom" : "universal",
+      "scale" : "1x"
+    },
+    {
+      "filename" : "github_light_2x.png",
+      "idiom" : "universal",
+      "scale" : "2x"
+    },
+    {
+      "filename" : "github_light_3x.png",
+      "idiom" : "universal",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}