Add recently viewed instances to "app global" settings

[mblomdahl]

On the Settings view, the current backend defaults (test data, Rebellion Tube and PeerTube Nighly) should be listed under a "Preset backends" page segment, but then we should have another segment labelled "Recent backends" with a list that the user can select from. The recent backend lists should be updated whenever a user adds a Sepia backend from #72 or navigates to an instance by opening a URL like "https://app.owntube.tv/web-client/?backend=videos.globenet.org" per #67.