Improve labeling for "Narrow by specific fields" on advanced search

[kimisgold]

This provides the following accessibility improvements for the "Narrow by specific fields" group on the advanced item search form:

• More detailed aria labels
• Visible labels for the individual selects and text inputs
• Alerts to screen readers when field rows have been added or removed

These updates are only implemented for the public views at the moment. I was thinking we could mirror the changes for the admin side once this is approved and merged, though open to doing that within this pull request as well.

[zerocrates]

Is the public.css that this adds being used anywhere?