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

datasette/views/index.py

@@ -21,7 +21,7 @@ class IndexView(RenderMixin):
             database = {
                 "name": key,
                 "hash": info["hash"],
-                "path": "{}-{}".format(key, info["hash"][:HASH_LENGTH]),
+                "path": self.database_url(key),
                 "tables_truncated": sorted(
                     tables, key=lambda t: t["count"], reverse=True
                 )[