Add account import thru polkadot.js extension

[Nexus2k]

Since polkadot.js extension supports accounts from many different sources (e.g. hard wallets from ledger, parity signer, etc.) it would be great to be able to import addresses from the extension instead of manually copy pasting.

[dcolley]

It could be an option to import from PJS and then create them in the DB.
Due to storage and indexing we need the address in the backend database.

[Nexus2k]

Yeah that would be fine. An import screen that shows all accounts respecting the network filter of the extension and then checkboxes for each account where user can choose which to import.