Add a button to delete local databases

[veloce]

Lichess app stores games and puzzles in local databases.

Add a button at the bottom of the settings screen to delete them. The button must show a confirm dialog.

EDIT

• [ ] display the amount of storage used in MB

[amanxnanda]

@veloce, I would like to work on this one.

[ASD-coder-design]

@veloce Could you please assign this issue to me

[tom-anders]

@veloce Could you please assign this issue to me

Someone else is already working on this: https://github.com/lichess-org/mobile/pull/827

Also, we don't assign issues in this project, see the Contributing guide