Fix/evenly assign quantifiers prevent overlap

[mattyg]

Resolves #535

• Verify that no quantifier has been assigned to same receiver multiple times, before generating assignments
• Refactor prepareAssignmentsEvenly to prevent a case where a quantifier could be assigned to the same receiver twice
• Improve doc comments explaining prepareAssignmentsEvenly

Post-Mortem
Previously in prepareAssignmentsEvenly we:

1. Generated a list of unique groups of receivers, duplicating each receiver for redundant assignments
2. Attempted to evenly distribute the groups of receivers among all quantifiers, based on the total number of praise in the group of receivers (i.e. a quantifier could be assigned to multiple groups of receivers).

This allowed a case where a quantifier could be assigned a duplicate receivers, because multiple groups of unique receivers could potential contain a duplicate between them.

With this refactor, we swapped the order of these steps. Now we:

1. Attempt to evenly distribute receivers among all quantifiers, based on the total amount of praise each receiver has (i.e. a quantifier could be assigned multiple receivers).
2. Duplicating the assignments, rotating the order, for each redundant quantification.

[sonarqubecloud]

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2.4% Duplication

[kristoferlund]

Great work @mattyg!! I have tested it as best I can with some different period/quantifier settings. Assignments look flawless!

I added a couple of small frontend additions to better handle error messages when quantifiers cannot be assigned.

Merging!