Add web app manifest file

CHDataManagement/Generator/HeaderElement.swift

@@ -50,11 +50,14 @@ enum HeaderElement {
 
     case robots
 
+    case manifest(FileResource)
+
     var order: Int {
         switch self {
         case .charset: 1
         case .robots: 2
         case .viewport: 3
+        case .manifest: 4
         case .icon: 10
         case .css(_, let order): order
         case .js: 20
@@ -79,6 +82,8 @@ enum HeaderElement {
             return file
         case .jsModule(let file):
             return file
+        case .manifest(let file):
+            return file
         default:
             return nil
         }
@@ -129,6 +134,8 @@ extension HeaderElement {
             return "<meta name='viewport' content='width=device-width, initial-scale=1, minimum-scale=1, maximum-scale=1' />"
         case .robots:
             return "<meta name='robots' content='noindex'>"
+        case .manifest(let file):
+            return "<link rel='manifest' href='\(file.absoluteUrl)'>"
         }
     }
 }
@@ -165,6 +172,8 @@ extension HeaderElement: CustomStringConvertible {
             return "viewport"
         case .robots:
             return "robots"
+        case .manifest:
+            return "manifest"
         }
     }
 }