Basic CSV export, refs #266

Tables and custom SQL query results can now be exported as CSV.

The easiest way to do this is to use the .csv extension, e.g.

	/test_tables/facet_cities.csv

By default this is served as Content-Type: text/plain so you can see it in
your browser. If you want to download the file (using text/csv and with an
appropriate Content-Disposition: attachment header) you can do so like this:

	/test_tables/facet_cities.csv?_dl=1

We link to the CSV and downloadable CSV URLs from the table and query pages.

The links use ?_size=max and so by default will return 1,000 rows.

Also fixes #303 - table names ending in .json or .csv are now detected and
URLs are generated that look like this instead:

	/test_tables/table%2Fwith%2Fslashes.csv?_format=csv

The ?_format= option is available for everything else too, but we link to the
.csv / .json versions in most cases because they are aesthetically pleasing.

tests/test_html.py

@@ -274,6 +274,21 @@ def test_table_html_simple_primary_key(app_client):
     ] == [[str(td) for td in tr.select('td')] for tr in table.select('tbody tr')]
 
 
+def test_table_csv_json_export_links(app_client):
+    response = app_client.get('/test_tables/simple_primary_key')
+    assert response.status == 200
+    links = Soup(response.body, "html.parser").find("p", {
+        "class": "export-links"
+    }).findAll("a")
+    actual = [l["href"].split("/")[-1] for l in links]
+    expected = [
+        "simple_primary_key.json",
+        "simple_primary_key.csv?_size=max",
+        "simple_primary_key.csv?_dl=1&_size=max"
+    ]
+    assert expected == actual
+
+
 def test_row_html_simple_primary_key(app_client):
     response = app_client.get('/test_tables/simple_primary_key/1')
     assert response.status == 200