[sql lab] option to disable cross schema search (#4551)

* [sql lab] disable cross schema search

This is killing our metastore as people type it emits large
all-table-dump as they hit the keystroke. It never returns as it times
out and hammers the poor metastore.

Also some improvements around the disabling the table select on the left
panel and having the table name not be sticky.

* typo

superset/assets/spec/javascripts/sqllab/SqlEditorLeftBar_spec.jsx

@@ -23,6 +23,7 @@ describe('SqlEditorLeftBar', () => {
     },
     tables: [table],
     queryEditor: defaultQueryEditor,
+    database: {},
     height: 0,
   };