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/table.py

@@ -750,7 +750,6 @@ class TableView(RowTableShared):
             )
             self.ds.update_with_inherited_metadata(metadata)
             return {
-                "database_hash": hash,
                 "supports_search": bool(fts_table),
                 "search": search or "",
                 "use_rowid": use_rowid,
@@ -851,7 +850,6 @@ class RowView(RowTableShared):
             for column in display_columns:
                 column["sortable"] = False
             return {
-                "database_hash": hash,
                 "foreign_key_tables": await self.foreign_key_tables(
                     database, table, pk_values
                 ),