Private key export

[Leo Wandersleb]

Allow exporting the account's private key on the settings page. By default it should be hidden.

• Show priv key in hex on settings page. MVP
• Clicking on a "show" button should reveal the hidden priv key. MVP+1
• Hitting on "copy" button should copy it to clipboard. MVP+1
• On mobile, the "copy" button should be a "share" button to allow avoiding the clipboard that is publicly available for all installed apps on Android MVP+2