Allow curators to add more than one annotation extension

[vanaukenk]

Curators may wish to add more than one extension to their standard annotation.

We need to add the ability to add more than one extension, either by having additional fields always available for entering data, or a button to click, as needed, to add another extension.

[tmushayahama]

is this for v1, this is actually on a difficult side and needs more careful thought. Since we are still using the activity unit backend. Things like cardinality. Doable but it will add complexity for v1 tagging @vanaukenk @thomaspd @kltm