Add anonymization feature for private user data

[codingjoe]

User privacy is important, not only to meet local regulations, but also to
protect your users and allow them to exercise their rights. However,
it's not always practical to delete users, especially if they have dependent
objects, that are relevant for statistical analysis.

Anonymization is a process of removing the user's personal data whilst keeping
related data intact. This is done by using the anonymize method.

[codecov]

Codecov Report

Merging #48 (3c2ffe0) into master (afa829f) will decrease coverage by 2.70%.
The diff coverage is 100.00%.

❗ Current head 3c2ffe0 differs from pull request most recent head fa125e9. Consider uploading reports for the commit fa125e9 to get more accurate results

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   99.68%   96.97%   -2.71%     
==========================================
  Files          18       23       +5     
  Lines         316      364      +48     
==========================================
+ Hits          315      353      +38     
- Misses          1       11      +10     

Impacted Files	Coverage Δ
mailauth/contrib/user/admin.py	100.00% <100.00%> (ø)
...0005_emailuser_email_hash_alter_emailuser_email.py	100.00% <100.00%> (ø)
mailauth/contrib/user/models.py	98.64% <100.00%> (+0.23%)	⬆️
mailauth/contrib/user/signals.py	100.00% <100.00%> (ø)
...th/contrib/user/migrations/0003_ci_unique_index.py	78.57% <0.00%> (-21.43%)	⬇️
mailauth/signing.py	91.42% <0.00%> (-8.58%)	⬇️
mailauth/forms.py	98.38% <0.00%> (-1.62%)	⬇️
mailauth/contrib/user/__init__.py	66.66% <0.00%> (ø)
mailauth/contrib/wagtail/__init__.py	66.66% <0.00%> (ø)
mailauth/contrib/admin/__init__.py	66.66% <0.00%> (ø)

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[herrbenesch]

@codingjoe nice feature.
I think the real blockers hereare that you are using digest as well as hexdigest for a field you want to filter on and that you are not populating the new hash field. Which, I guess, is something you want.
I surely can't know all the implications this change will have on actual usage. Perhaps we should once install it @ voiio platform and test.

[codingjoe]

@Fidelitas I decided to ditch the whole hash thing. Hash collisions are an unlikely problem, but there really is no need to retain a hash. So just anonymization now :)

[lgtm-com]

This pull request introduces 1 alert when merging 16f38aa into 8d3634e - view on LGTM.com

new alerts:

• 1 for Conflicting attributes in base classes

[lgtm-com]

This pull request introduces 1 alert when merging 03cfcad into 8d3634e - view on LGTM.com

new alerts:

• 1 for Conflicting attributes in base classes

[herrbenesch]

I'm really impressed of the clean code (also the tests) that has been made here.
Very well done!

[lgtm-com]

This pull request introduces 1 alert when merging 3c2ffe0 into 8d3634e - view on LGTM.com

new alerts:

• 1 for Conflicting attributes in base classes

[lgtm-com]

This pull request introduces 1 alert when merging 8868e3d into afa829f - view on LGTM.com

new alerts:

• 1 for Conflicting attributes in base classes

[lgtm-com]

This pull request introduces 1 alert when merging fa125e9 into afa829f - view on LGTM.com

new alerts:

• 1 for Conflicting attributes in base classes