Pinning integration: Stub UI for Settings screen

[jessicaschilling]

Note: This issue is part of a larger pinning service integration epic undertaken spring/summer 2020.

In brief

Stub out UI for Settings screen of Web UI/Desktop, as follows:

• Create high-fidelity Figma doc displaying all steps in process, including errors/modals/etc
• Build in the display (but not functionality) of new Settings screen elements as depicted in Figma
• Add the minimum amount of interactivity required to be able to travel through the Files screen to demonstrate said elements
• Create required documentation (to be homed on docs.ipfs.io) as linked to from within the UI

High-res screens as Figma doc are here.

[jessicaschilling]

@rafaelramalho19 @lidel Per discussion earlier today -- Updated Figma high-res screens to include the full settings flow:

• Screen: First load/no third-party services
• Modal: select service provider
• Modal: configure known service provider
• Modal: configure custom service provider
• Screen: local pinning plus several third-party service providers
• Pop-up: dots menu for a service provider
• Modal: remove a service provider
• Screen: confirm service provider was removed

Would appreciate feedback/discussion with you both before we take this wider. Thanks!