Provide a QR code of private key for ease of use

**Describe the usability problem**
Some text scanning software is not capable of discerning lower case `b` from a `6` and this can produce undesirable results if a user relies on this to deal with moving private keys from extension into mobile.

**Screenshots**

Private key in extension today:

SRP in extension today:

**Proposed behavior**
This ticket is to request that we make available to the user a QR rendering of the private key, similar to how private key is offered today on mobile and how SRP is handled today in both mobile and extension.

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_filled out after submission_

**User impact | Usage frequency | Feasibility scores**
_How important is this to the user, what the user wants to accomplish | Daily, weekly, monthly | Small, medium, large_

Compare the extension's current private-key presentation with the SRP presentation described in the issue and review the corresponding extension UI entry points. The work is complete when users can access a QR rendering of the private key, while preserving the existing private-key and SRP flows.

Written by the indexing model from the issue text.