Make MyNearWallet behave like Meteor

[gagdiez]

Currently, My NEAR Wallet redirects the user to the MNW site each time the user needs to interact with the wallet (login, sign)

We need to implement a couple of changes, so it opens in a new page (as Meteor does), thus not interrupting the user flow

Objectives

• Modify the wallet-selector/ my-near-wallet package so it opens MNW in a new pop-up window
• Make it exchanges messages with the child window, without reloading the original app
• Implement sign and submit tx
• Implement sign message
• Modify MNW page to be able to interact with the wallet-selector/ my-near-wallet package