Refactor & Extract image viewers (#1074)

* Extract `ImagePreviewView` + add `#Preview`

* Extract image pickers + dedupe and bg processing of images

* Include a dummy photo in the preview assets vs live url

* Fix development assets + url percent encoding

* Fix iOS vs macOS build issue

* Fix SPM-related issues

bitchat/_PreviewHelpers/PreviewAssets.xcassets/Contents.json

@@ -0,0 +1,6 @@
+{
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

bitchat/_PreviewHelpers/PreviewAssets.xcassets/dummy.imageset/Contents.json

@@ -0,0 +1,21 @@
+{
+  "images" : [
+    {
+      "filename" : "dummy.jpg",
+      "idiom" : "universal",
+      "scale" : "1x"
+    },
+    {
+      "idiom" : "universal",
+      "scale" : "2x"
+    },
+    {
+      "idiom" : "universal",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}