Handle backup message type with different user texts in siwe.reac.js

Summary: This differential modifies SIWE component so that it renders different user texts based on the type of message we want to sign.

Test Plan:
It may sound a little strange:

1. Modify fullscreen-siwe-panel.react.js/connect-ethereum.react.js to provide SIWEMEssageTypes.MSG_BACKUP instead of MSG_AUTH.
2. Attempt/log in registration process.
3. Ensure that log in/registration is succesfull but the texts that are displayed refer to backup message generation.

Reviewers: tomek, atul, inka

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11635