Implement a onboarding screen to explain the current version to new users

[nazrinharris]

Would need a user-independent database to store whether the user has been to the onboarding screen or not. Since I'm planning to introduce a user-dependent database in the next PR, I believe using another storage solution would be the simplest way forward. Specifically I'll be using shared_preferences.

Deed

• [x] Add shared_preferences.
• [x] When user finishes the splash intro, a key (is_first_open) would be set to false.
• [x] Allow the user to walk-through onboarding screen multiple times from the StartScreen
• [x] Enhance the is_first_open check and shared_preferences into a standalone feature of sorts.

[nazrinharris]

Should be complete with f8817c8 in #118