Support relationship entity references

Problem/Motivation

We had a relationship on a field on a taxonomy term, which was an entity reference to another taxonomy term. This wasn't supported by the module, and as such we always received empty filters when we should have got some.
I've created a patch for this, which checks if the relationship is another entity, and also accounts for it being multivalue.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes