Note text field for venues

As discussed in #9438, it would be helpful if a venue page could display a note that it is related to some other venue, for example.

No files or tests are named in the issue. Start by locating the venue data model and the path that renders a venue page, then see how venue fields are passed into the template. Add or wire a note field in that flow so related-venue text can be displayed on the page. Done means venue pages render the note and existing venue fixtures/tests for page rendering pass with the new field.

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