Manage Authorities: the search term and authority type fields need to be associated with their labels

To recreate:
* Install a browser extension like WAVE or aXe
* In the staff client, go to Cataloging > Manage Authorities
* Run your accessibility extension.

WAVE flags the "Missing Form Label" error twice. aXe considers this as two separate issues: "Form elements should have a visible label" and "Form elements must have labels".

In any case, for both "Search Term" and "Authority Type" fields, there is a label visible in the screen, but it is not programmatically associated with the field it describes. We should:
1. On the Search Term input, change the erroneous "aria-describedby" attribute to "aria-labelledby".
2. Give the Authority Type combobox a domId.
3. Turn the Authority Type <span> "label" into an actual <label> tag with the new domId value in the for attribute.

More info from WebAIM: https://webaim.org/techniques/forms/controls