Add keys migrate command

[alessio]

Add new command to assist users migrate their keys from the legacy
on-disk keybase to the new OS keyring-based implementation.

Ref #4754

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[fedekunze]

@alessio tests are failing

[alessio]

Fixed @fedekunze, thanks!

[codecov]

Codecov Report

Merging #5097 into master will increase coverage by 0.01%.
The diff coverage is 57.35%.

@@            Coverage Diff             @@
##           master    #5097      +/-   ##
==========================================
+ Coverage   54.86%   54.87%   +0.01%     
==========================================
  Files         295      296       +1     
  Lines       18261    18336      +75     
==========================================
+ Hits        10019    10062      +43     
- Misses       7493     7524      +31     
- Partials      749      750       +1

[fedekunze]

thanks @alessio

[alessio]

Once this is merged, I'll add all remaining commands in an ensuing PR. Once the latter is done, then I'll get on with the following (note: this comment was added by @alexanderbez):

// 1. Remove lazyKeybaseKeyring and make keyringKeybase useable by the outside
// world.
// 2. Mark all keyringKeybase methods private and remove the compile time
// Keybase interface assertion.

EDIT: @alexanderbez comment is now being addressed in #5104

[alexanderbez]

huh? Shouldn't we export with a user's encryption passphrase instead of hard-codec "abc"?

[alexanderbez]

tested ACK 🎉