Replace the settings page with a preferences window

[Mongey]

Dé Céadaoin, mo chara.

4 years late to the party, but, addresses #3, #5, #10

• Allows you to follow different organisations. ( e.g nolaneo/SeaEye && mongey/shitehawk)
• Allows you to add multiple clients. (e.g. CircleCI Enterprise && Public)
• Allows you to toggle notifications per-project. ( Partially addresses Allow configurable notifications #29 )

👷 work in progress👷

[benoittgt]

I still think it should be merged :)