Prepare a device for cloning: Management command for clearing user data

[benjaoming]

Summary

Fixes #5341

This new command is essential for writing a good new RPI deployment guide.

TODO

If not all TODOs are marked, this PR is considered WIP (work in progress)

• Have tests been written for the new code? If you're fixing a bug, write a regression test (or have a really good reason for not writing one... and I mean really good!)
• Has documentation been written/updated?
• New dependencies (if any) added to requirements file

[benjaoming]

Self-assigning. I've tested the command, and it works :)

Will start to write the Raspberry Pi guide for 0.17 now.

[codecov-io]

Current coverage is 51.11% (diff: 0.00%)

Merging #5343 into develop will decrease coverage by 0.07%

@@            develop      #5343   diff @@
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  Files           141        142     +1   
  Lines          7447       7472    +25   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           3812       3819     +7   
- Misses         3635       3653    +18   
  Partials          0          0          

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[jamalex]

Belated ❤️