Add onboarding swiper as welcome screen

Our current welcome screen before a user logs in:

I quite like it, but I think having a welcome swiper explaining the different things the app could do would be even better 🤓

For example: the Slack mobile app

The Product Hunt app

Doesn’t have to be as anything as complicated as those options - they’re just for reference.

Some points however:

1. react-native-swiper seems to be an excellent library that can do just this
2. I like the idea of having the Sign In button at the bottom always so even though the user can continue to swipe, they can always just click the sign in button
3. I’ll leave the floor open for design suggestions. Something simple with a top title (Control your notifications for example, a GitHub octicon in the middle and a mini blurb below (View and control all of your unread and participating notifications. for example is what I’m envisioning. Feel free to just lift copy from the landing page

Always open to further ideas and suggestions of course ❤️

[housseindjirdeh]

Current progress

Will have a PR up for it tomorrow (needs a little more color I think 🤔)

I’m thinking the slides will be in this order:

1. Welcome
2. Notifications
3. Repository/User
4. Complete issue and pull request control

Let me know if anyone has any suggestions!

[lex111]

I would also list the main features, add more icons, and maybe make an animation?