Various checks when importing validator account keys

[stefa2k]

Before importing a validator account key there should be certain checks to ensure no slashing can occure:

• [ ] Look on beaconcha.in if the validator account key is offline for the last 2 epochs
• [x] Look up if there is a slashing protection file provided #629

Notify the user about any failed checks. The user can still import when confirming about the failed checks.

[daGscheid]

// change plans to get Data from Beacon API

[gbayasgalan]

// change plans to get Data from Beacon API

works with up to 10 keys for now using beaconcha https://github.com/stereum-dev/ethereum-node/pull/1084