[EXPORT] Minor fixes to the export form

[colemanw]

Overview

Couple of minor fixes to the export form:

1. Only show enabled contact types (only relevant if e.g. Households are disabled)
2. Correctly show list of relationship types for a given contact type.

Before

After

Comments

This is mostly to support the new ExportUi extension. This does cause the existing export form to burp up an e-notice if you disable a contact type like Households or Organizations, but I I'm not sure that's a big problem, as that scenario isn't actually supported in the UI and it'll be replaced by the extension soon (which handles it fine with no e-notice).

[civibot]

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[eileenmcnaughton]

this makes sense in the export UI (screenshots added) - just checking import & search builder

[eileenmcnaughton]

OK I checked import & the extra relationships don't make sense there either & search builder seems unaffected.

Relationships like Child -> Parent still show twice but we no longer see 'Head of Household is' - which only makes sense for Households. I think I hit confusion on this in the past so it's good to change. Merging