Support constant values in relationships

### Is your proposal related to a problem?
Trying to create relationships to polymorphic tables is really frustrating, the only good solution is to create a view for every polymorphic table

### Describe the solution you'd like
Support an option to set a constant value when creating a relationship.
As for the type of the value, best if it would be inferred from the type of the field it is compared to, but even selecting the field type in the UI is great.

The issue names no files, tests, or entry points. Start by tracing where relationships are created in the UI and how the compared field type is represented; done would mean a relationship can use a configured constant value, with its type inferred or selectable as described.

Written by the indexing model from the issue text.