URL hashing is now off by default - closes #418

Prior to this commit Datasette would calculate the content hash of every
database and redirect to a URL containing that hash, like so:

    https://v0-27.datasette.io/fixtures => https://v0-27.datasette.io/fixtures-dd88475

This assumed that all databases were opened in immutable mode and were not
expected to change.

This will be changing as a result of #419 - so this commit takes the first step
in implementing that change by changing this default behaviour. Datasette will
now only redirect hash-free URLs under two circumstances:

* The new `hash_urls` config option is set to true (it defaults to false).
* The user passes `?_hash=1` in the URL

tests/test_utils.py

@@ -59,6 +59,13 @@ def test_path_with_removed_args(path, args, expected):
     )
     actual = utils.path_with_removed_args(request, args)
     assert expected == actual
+    # Run the test again but this time use the path= argument
+    request = Request(
+        "/".encode('utf8'),
+        {}, '1.1', 'GET', None
+    )
+    actual = utils.path_with_removed_args(request, args, path=path)
+    assert expected == actual
 
 
 @pytest.mark.parametrize('path,args,expected', [