Standardise layouts across different app screens

The https://www.figma.com/file/d6n3MPVRWbL5jP2NmvW96b/De_Urgenta?node-id=1466%3A530 has some parts that need to be adjusted in order to make the app respect the android material guidelines.

We could:
- use the app bar to display all screen headers / titles
- replace the `x` button with a more common `<` in the appbar for back navigation between fragments
- replace images with proper vector icons (we should request all needed icons on slack)
- standardise colors and text sizes fixed in https://github.com/code4romania/de-urgenta-android/pull/85