[CNTools] - Add mnemonics when creating wallet (for offline mode only)

[rdlrt]

Feedback from the guild members:

• Make it available in offline mode only
• Ensure to not log anywhere, or store in variables that are reusable
• Add a basic test with ping against github.com and warn users if their machine is online.
• Add validation of mnemonics saved

Doco:

• Re-inforce disclaimers about responsibility of keys, as well as ownership and security of their device

[rdlrt]

Depends on what we decide with #587