HealthGate should be HealthOverlay

[amaury1093]

Right now, we have one HealthGate, which waits for the node to be sync before displaying any children. Once it's sync, one child is the ApiContext, which injects api everywhere.

But some stuff, like account creation, doesn't require ApiContext. So account creation screens should be accessible even if the node is still syncing.

We should instead do more or less like Fether: i.e. multiple overlays

• <RequiredNodeConnected>: provider is able to ping the node
• <RequiredSynced>: node is synced
• <RequiredApiReady>: api has emitted isReady event

And most importantly, they should be overlays, aka still render the children.