Streamline link creation if only one type of link is allowed

[GuySartorelli]

Developers can set which types of links are allowed to be created for a given link field.

If only one type is allowed to be created, the "Add Link" dropdown should become a button that says "Add [type] Link" (e.g. "Add Phone Link"). Clicking it should directly open the modal for creating that link type.

[maxime-rainville]

Cassie thought this would be valuable, but not overly important to the experience.