UI for choosing profile picture

Add an item to the settings screen that shows the user's current profile picture (if any) with a button for choosing a new picture.

Create a new confirmation screen that shows the chosen picture cropped to a circle, with an explanation that the picture will only be visible to contacts and buttons to confirm or cancel the choice.

Depends on #1844 (closed), #1846 (closed). Subtask of #214 (closed).