Add a SignIn Activity

[aayushsingla]

A signIn activity is required at startup because posting a pic in photoUploadFeed and retrieving data from drive requires user to be authenticated.Also, users data(such as: name and pic) need's to be retrieved.

[Rushi98]

Need not be at start-up, require login only when user chooses to upload

[aayushsingla]

aren't we going to keep a signIn at start? No one will post in feed if they will have to signIn specifically for that and we might need signIn for other features too.

[Rushi98]

If a user is reluctant to login, he will not use app if he's required to login at start without any obvious reason for requirement of his personal data. If we prompt him to login at data specific features, we have better chances of convincing him and he will anyway be using other features of app even if he is hell bent on refusing to identify himself. Don't be hostile to user.