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/fixtures.py

@@ -73,6 +73,13 @@ def app_client_no_files():
     yield client
 
 
+@pytest.fixture(scope="session")
+def app_client_with_hash():
+    yield from make_app_client(config={
+        'hash_urls': True
+    })
+
+
 @pytest.fixture(scope='session')
 def app_client_shorter_time_limit():
     yield from make_app_client(20)