Change email for mystnodes

[Peter-SovietSquirrel]

Currently the only option if a user wishes to change their registered email is to sign up under that email and then re-import all their nodes.
A better option would be to allow users to change their registered email address.

[archerallstars]

I vote for this, as Skiff, the email I used to sign up the node is going to demise in 6 months. This sort of thing could happen.

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For anyone wanting to change their email:

1. Sign up with a new email.
2. Grab your new API key on https://mystnodes.com/nodes.
3. On the node machine, run myst config show | grep mmn.api-key to see your current API key registered with your node.
4. Run myst config set mmn.api-key <your new API key>. You might want to re-check the key with the previous command.
5. Run sudo systemctl restart mysterium-node.service to restart your node. It will run with a new config. You should be able to see your node on a new account.