Standardise layouts across different app screens

[aniri]

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 Bugfix/firebase deploy action and styles #85

[lukstbit]

replace images with proper vector icons (we should request all needed icons on slack)

We need a vector asset for the logo plus vector assets for brand logos used in the app(Apple, Facebook, Google and Github).