Welcome modal

Adds new modal for app tour for users

The blur effect does not work in Android, I don't think there are alternative for expo, modules available require changes in native code...

• styles in ModalStyle.js
• Slides.js for rendering slides inside the modal
• Modal integrated into App.js

Live preview at https://exp.host/@yashovardhanagrawal/crowdAlert

@sidd607 @anujgupta61 I have used a different way in the PR, instead of a whole dedicated screen, a modal works here