UI update/amount picker

[soerenbf]

Purpose

Implementation of amount picker to be used for transfer flows + validation/delegation flows.

To test the component it's easiest to use storybook (which was upgraded recently so remember to re-install):

yarn && yarn storybook

Changes

• Fix storybook viewport sizes to the scales used in the wallet (+ fullscreen)

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.