Refactor hydration and serialization functions

[oskar-codes]

This pull request refactors the existing hydration functions and adds serialization functions to User, Association and Event. This will ensure data is properly formatted when sending/retrieving data from Firestore.

This follows this comment on #32.

[oskar-codes]

The 3 previous commits further enhance this pull request with additions concerning error handling and testing. I will now merge this branch.

[oskar-codes]

We really need to find a way to block pushing if the files aren't formatted using ktfmtFormat.

[sonarcloud[bot]]

Quality Gate passed

Issues
9 New issues
0 Accepted issues

Measures
0 Security Hotspots
90.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud