Extension: Connection Approval should provide warning text to user

[jurevans]

When connecting to the extension, the user should be informed as to what information will be shared with the connected interface, such as:

• Public (unencrypted) account data (address, public key, alias)
• Viewing keys (for shielded accounts)
• Proof-generating keys (for shielded accounts)
• Ability to sign in extension with these keys