[UX] Contact module: Rename "categories" to "recipient groups" in the admin UI - allow customizing the label of the field in the public-facing form

DRAFT PROPOSAL

This is what site admins see in the UI:

This is what the people filling the form see:

I would like to propose the following changes: - Rename "categories" to "recipient groups" in the backend, since that is what people are actually creating. - Allow customizing the label of the "Category" field (what site visitors see in the form). Depending on the use case, you may want to call it "Reason for contacting us", or "Select department to contact", or whatever. - Whatever the label is configured to be, make that the heading of the "Category" column. - Potentially also allow customizing the label of each "category", so that the options shown to site visitors in the select can have different labels from what the admins call them "internally".