[chore] extract reusable header component

[devKiratu]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

The reusable MainHeader component is extracted from similar header code across Screens, thus removing code repetition. This fixes #45

It renders differently based on whether a user signed in or not. It is shared by the following screens:

HomeScreenNotLoggedIn.tsx   
HomeScreen.tsx   
FeedScreen.tsx
AboutScreen.tsx

The MainHeader component also fixes #44 ( header components shifting off screen)

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] I have run the project locally and it works on both the Android emulator and an android device running on Android 9
• [ ] Test B

Screenshots:

Home Screen

Signed out

Signed in

Feed Screen

About Screen

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[devKiratu]

@brianwachira please review

[devKiratu]

Good work @devKiratu

Thanks!