Enhancement/#3 linking second device

[claasklar]

Contributor checklist:

• [x] My commits are in nice logical chunks with good commit messages
• [x] My changes are rebased on the latest development branch
• [x] A yarn ready run passes successfully (more about tests here)
• [x] My changes are ready to be shipped to users

Description

This PR enables to link a Signal-Desktop device to another one. There are two new major views, the manage_devices_view and the add_device_view. In the former one devices can be removed and in the second view new ones can be added. To deal with the device management both the AccountManager and the WebAPI had to be extended.

Resolves #3