Create RelationFilterMetadata

[FelixMalfait]

We should be able to set default filter on relations.
One use-case is for the opportunities field, we shouldn't be able to assign a person that is from a different company than the company attached to the opportunity.
Another use-case would be to create a field FutureTasks which only show tasks in the future.

I think we should create a dedicated table relationFiltersMetadata.
With columns: relationMetadataId, toFieldMetadataId, operand, filterValueFieldMetadataId, filterValueRaw

fieldMetadataId is for the left side of the operand
the right side can be either a fixed value (value column) or another field in the same table (valueAsAFieldMetadataId)

In the case of an opportunities' point of contact, we would add a single Filter record with values:

• relationMetadataId: id of the opportunities <-> people relation
• toFieldMetadataId: point of contact field
• operand: eq (equal)
• filterValueFieldMetadataId: id of the company field on opportunities

This will require an additional decorator on the backend similar to RelationMetadata decorator.
We then need to make sure we then apply those filters automatically (doing it on the backend would be best)

(Closing #2632 which was related to start fresh)