SelectControl hackery: Fixing freeform select and allowing group by and non group-by column selectors to take in arbitrary expressions including select-all semantics. (#6722)

Summary: We want to allow grouping by on expressions, including the
ability to select expressions (without group-by). The UI ain't good yet
.. it is not at the feature parity of say adhoc filters/metrics that
come with a nice text-box to edit the SQL. But
this suffices for my usecase for now.

(The UI would have to be redone ... but that would require some refactoring
to merge the AdhocFilter/Metric/Popover stuff such that it can be used
for this use case too.)

Also fixed a bug in the SelectControl freeForm selection.

Allow selecting all columns easily: Added a new Select-All special item
that users can select to make all the options in the selectable show
up.

The ability to group by arbitrary expressions is useful because now two
users don't need to create custom computed-fields for this.

tests/base_tests.py

@@ -26,6 +26,7 @@ from superset import app, db, is_feature_enabled, security_manager
 from superset.connectors.druid.models import DruidCluster, DruidDatasource
 from superset.connectors.sqla.models import SqlaTable
 from superset.models import core as models
+from superset.models.core import Database
 from superset.utils.core import get_main_database
 
 BASE_DIR = app.config.get('BASE_DIR')
@@ -99,6 +100,9 @@ class SupersetTestCase(unittest.TestCase):
     def get_table_by_name(self, name):
         return db.session.query(SqlaTable).filter_by(table_name=name).one()
 
+    def get_database_by_id(self, db_id):
+        return db.session.query(Database).filter_by(id=db_id).one()
+
     def get_druid_ds_by_name(self, name):
         return db.session.query(DruidDatasource).filter_by(
             datasource_name=name).first()