APIv4 Explorer - UI support for join ON clause

[colemanw]

Overview

Improves the APIv4 explorer to give selectable clauses when adding a JOIN.

Before

Adding a JOIN would give you a textfield to enter the code of your ON clause. Good luck!

After

Adding a JOIN gives you an interactive clause group similar to WHERE or HAVING.

[civibot]

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[eileenmcnaughton]

Affects explorer only - we are still treating apiv4 as being in 'aggressive development mode' so only did a light check

[colemanw]

Thanks @eileenmcnaughton