Respect _size on arbitrary SQL query pages

[MatrixHackin]

Summary

This fixes #1200 by making _size= work on arbitrary SQL query pages.
Previously, URLs such as:

/fixtures/-/query?sql=select+*+from+compound_three_primary_keys&_size=10

ignored _size=10 and displayed all rows up to max_returned_rows. But the query result with many figures may cause rendering pressure.This change:

• parses and validates _size in QueryView
• supports _size=max
• limits arbitrary SQL query results according to _size
• marks results as truncated when _size hides additional rows
• persists _size in the query form using a hidden input

Tests

conda run -n datasette-issue1200 pytest -q \
  tests/test_api.py::test_custom_sql_size \
  tests/test_api.py::test_custom_sql_size_validation \
  tests/test_html.py::test_query_size \
  tests/test_api.py::test_custom_sql \
  tests/test_html.py::test_query_json_csv_export_links

Result:

7 passed