.csv should link to .blob downloads

[simonw]

• Update .csv output to link to these things (and get that xfail test to pass)
• Add a .csv?_blob_base64=1 argument that causes them to be output in base64 in the CSV

Moving the CSV work to a separate ticket.
Originally posted by @simonw in #1061 (comment)

[simonw]

Remove this xfail and import pytest:

datasette/tests/test_csv.py

Lines 83 to 96 in 503a5b7

	@pytest.mark.xfail
	def test_table_csv_blob_columns(app_client):
	response = app_client.get("/fixtures/binary_data.csv")
	assert response.status == 200
	assert "text/plain; charset=utf-8" == response.headers["content-type"]
	assert EXPECTED_TABLE_CSV == textwrap.dedent(
	"""
	rowid,data
	1,/fixtures/binary_data/-/blob/1/data.blob
	2,/fixtures/binary_data/-/blob/1/data.blob
	""".strip().replace(
	"\n", "\r\n"
	)
	)

[simonw]

This will close #1034.

[simonw]

I'm not going to do the base64 thing unless someone asks for it.