Simplify Backup Page

[rolznz]

Currently the backup page is scary and complicated (requires saving multiple things - see #750).

We could split it into Alby Account vs no Alby Account components to reduce the conditionals.

No Alby Account could have a button to download SCBs (or somewhere else, e.g. on the node page which is shown whenever a new channel is opened and user does not have backups for that channel yet).

Alby Account version can be improved when we implement VSS and remote backup of NWC data.

[jpl-btc]

Just an idea: For no-Alby Account Hubs, whenever a new channel is opened, include a prompt or a warning somewhere reminding the user to download the SCB (Static Channel Backup) file. It might be a little annoying for power users, but increases the user's security by pushing him/her to download the necessary backup files after each new channel, as it should be, and they may better understand the convenience of having an Account and a subscription.

[rolznz]

This could also be good for Alby Account users who still want to save SCBs locally ("what happens if alby disappears" question).