Make instance switcher a page instead of a modal

This is a preparation for #2089 (closed).

Since the Desktop application is not shipped using the web, we cannot automatically determine the instance URL. We therefore need to have a instance selection as the entry point into the App. The instance switcher is a great starting point, although it probably needs some improvements. But its uncool to have it as a modal, so we need to make it a page. For the usual instance switch it doesn't matter anyways, since its only used in development and after switching the instance, you get forwarded to the starting point of the webapp anyways, so the current page doesn't need to be preserved.

@wvffle already drafted code to load this page for the Desktop app on startup:

if (typeof window.__TAURI_IPC__ !== 'undefined') {
  whenever(() => !store.instanceUrl, () => {
    router.push('/connect')
  }, { immediate: true })
}

Optional (Can be moved to another issue):

We can fetch instances from network.funkwhale.audio in order to provide some defaults for the user to connect to.