Notifications tab

Created by: Wv5twkFEKh54vo4tta9yu7dHa3

TODO:

• Add fallback image for thumbnail

• remove thumbnail image for notifications without thumbnail to show (follow, etc)

• open relevant activity instead of browser (depends on other PR #28 and making an activity for seeing the profile of someone else (@mjaillot))

• tests

• dynamically load notifications (lazy loading)

• cache into database (@Sanimys ?)