Implement unit testing
This MR merge developed unit testing tests for most some screens, after merge with !7 (merged), will also incorporate test stage on Gitlab CI/CD.
It also implements necessary generated and coded mocks.
Some components tested:
- Post Teaser Widget
- File Download Card Widget
- Post Bookmark Widget
- Post Header Widget
- Publish Form Widget
- Login Screen
- Splash Screen
- Post Details Screen