fix(app): resize app icon renditions to their required exact pixel sizes

All 10 macOS AppIcon slots (16 through 1024) were the same untouched
1254x1254 source image, so Xcode rejected every one of them ("is
1254x1254 but should be NxN"). Resized each with sips to its exact
required size, and filled in the previously-empty iOS 1024x1024 slot
with an alpha-flattened copy (iOS icons can't have transparency;
macOS renditions keep it).

Outpost/Assets.xcassets/AppIcon.appiconset/Contents.json

@@ -1,6 +1,7 @@
 {
   "images" : [
     {
+      "filename" : "outpost-ios-1024.png",
       "idiom" : "universal",
       "platform" : "ios",
       "size" : "1024x1024"
@@ -28,31 +29,37 @@
       "size" : "1024x1024"
     },
     {
+      "filename" : "outpost 9.png",
       "idiom" : "mac",
       "scale" : "1x",
       "size" : "16x16"
     },
     {
+      "filename" : "outpost 8.png",
       "idiom" : "mac",
       "scale" : "2x",
       "size" : "16x16"
     },
     {
+      "filename" : "outpost 7.png",
       "idiom" : "mac",
       "scale" : "1x",
       "size" : "32x32"
     },
     {
+      "filename" : "outpost 6.png",
       "idiom" : "mac",
       "scale" : "2x",
       "size" : "32x32"
     },
     {
+      "filename" : "outpost 5.png",
       "idiom" : "mac",
       "scale" : "1x",
       "size" : "128x128"
     },
     {
+      "filename" : "outpost 4.png",
       "idiom" : "mac",
       "scale" : "2x",
       "size" : "128x128"