Very aggressive legacy note, resolves #221 (#862)

src/html/index.pug

@@ -39,6 +39,7 @@ html
     include modals/error
     include modals/password
     include modals/oem-lock
+    include modals/legacy-android
     include modals/low-power
     include modals/select-device
     include modals/developer-mode

src/html/modals/legacy-android.pug

@@ -0,0 +1,13 @@
+#legacy-android-modal.modal.fade(tabindex='-1', role='dialog')
+  .modal-dialog(role='document')
+    .modal-content
+      .modal-header
+        button.close(type='button', data-dismiss='modal', aria-label='Close')
+          span(aria-hidden='true') ×
+        h4.modal-title This is a legacy Android device!
+      .modal-body
+        p If your device is running Android, you might have to #[a(onclick="shell.openExternal('https://docs.ubports.com/en/latest/userguide/install.html#install-on-legacy-android-devices')") install the old Ubuntu Touch version from Canonical] first to unlock the bootloader.
+        p #[b The UBports Installer will not work while this device is running Android!]
+        p You can follow #[a(onclick="shell.openExternal('https://docs.ubports.com/en/latest/userguide/install.html#install-on-legacy-android-devices')") this step-by-step guide] to unlock your device.
+      .modal-footer
+        button.btn.btn-default(type='button', data-dismiss='modal') Got it, my device is unlocked!

src/html/scripts/main.pug

@@ -157,6 +157,8 @@ script.
         $("#device-name").text(output.name);
         if (isLegacyAndroid && !ubuntuCom)
           $("#legacy-android-note").show();
+        if (isLegacyAndroid)
+          modals.show("legacy-android");
         if (autoDetected)
           footer.underText.set("Are you ready for Ubuntu Touch on your " + output.name + "?");
         else