Add caching of current user

[abitofevrything]

Description

Closes #542.

After thinking about it, I've decided that the only missing entity was the current user - so I just added a way to specify that.

The entities are partial so we can potentially move to an ID-based system in the future (and because the entities won't be updated).

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works