Implement floating action button with menu

[thomaseizinger]

In order to give the user more choices in what to add the nobt, a proper floating action button should be implemented as per material design spec. Currently, only bills can be added to the nobt but later on, users should also be able to add payments in order to settle debts for example.

[thomaseizinger]

Initial implementation of such a menu can be found here: https://github.com/thomaseizinger/frontend/tree/feature/fabmenu

There a still a few problems:

• [x] Inconsistent animation behavior in FF and Chrome
• [x] The current design of the home-screen does not allow a transparent overlay over our header. Without that, the overlay that is used to visually separate the button choices from the background looks a bit odd.
• [x] Back-Button on mobile does not yet close the menu.
• [x] Click outside does not yet close the menu.