Make sure users know where linked vocab terms will take them

[jggautier]

In a recent demo of external vocabulary, we looked at how the terms that dataset depositors choose appear on the dataset page when those terms include a URI.

[TaniaSchlatter]

We want to help users know where links will go before they click them. For context, there is a future use case for linking metadata terms to related datasets. However, in the near term, since all links will go offsite, @jggautier and I suggest adding a line of gray help text at the top of the block above the form fields: "Clicking on link below will take you offsite to more information about the term." (not shown in the example image)