Add User object to DMs per Issue #344

[jeremylow]

Description

Adds a full-hydrated User object to the DirectMessage object for the sender and recipient properties.

Related Issue

#344

How Has This Been Tested?

Tests added for DirectMessage Model. All prior tests pass.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

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This change is

[bear]

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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