User deletion

[jyotigautam]

Why?

To allow user deletion in the system.

This change addresses the need by:

1. Adding state (active or deleted) and deleted_at field to user and allowing soft deletion of user.
2. Add unique index to user email and deleted_at field in user schema.

Migration added: apps/snitch_core/priv/repo/migrations/20190128081011_update_user_unique_index.exs

Referred blog: Make soft deleted user email available again

delivers #163182381

Checklist

• [ ] I have read CONTRIBUTING.md.
• [ ] My code follows the style guidelines of this project.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] My code does not generate any (new) credo and compile-time warnings.
• [ ] I have updated the documentation wherever necessary.
• [ ] I have added tests to cover my changes.

[codecov[bot]]

Codecov Report

Merging #451 into develop will increase coverage by 0.13%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #451      +/-   ##
===========================================
+ Coverage    77.73%   77.86%   +0.13%     
===========================================
  Files          174      174              
  Lines         2120     2128       +8     
===========================================
+ Hits          1648     1657       +9     
+ Misses         472      471       -1

Impacted Files	Coverage Δ
...pps/snitch_core/lib/core/domain/account/account.ex	100% <ø> (ø)	:arrow_up:
apps/snitch_core/lib/core/data/model/user.ex	100% <100%> (+11.11%)	:arrow_up:
apps/snitch_core/lib/core/data/schema/user.ex	100% <100%> (ø)	:arrow_up:
...nitch_core/lib/core/tools/ecto_enums/ecto_enums.ex	100% <100%> (ø)	:arrow_up:

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[ashish173]

@jyotigautam There are some cases we need to test for user model.

1. The scenario where we create -> delete a user -> create again with same email = result should be two different user records with the same email but only the active user is returned in model functions.