[AIRFLOW-3067] Display www_rbac Flask flash msg properly (#3903)

The Flask flash messages are not displayed properly.

When we don't give a category for a flash message, defautl
value will be 'message'. In some cases, we specify 'error'
category.

Using Flask-AppBuilder, the flash message will be given
a CSS class 'alert-[category]'. But We don't have
'alert-message' or 'alert-error' in the current
'bootstrap-theme.css' file.

This makes the the flash messages in www_rbac UI come with
no background color.

This commit addresses this issue by adding 'alert-message'
(using specs of existing CSS class 'alert-info') and
'alert-error' (using specs of existing CSS class 'alert-danger')
into 'bootstrap-theme.css'.

airflow/www_rbac/views.py

@@ -2231,10 +2231,9 @@ def action_set_running(self, drs, session=None):
                 dr.state = State.RUNNING
             models.DagStat.update(dirty_ids, session=session)
             session.commit()
-            flash(
-                "{count} dag runs were set to running".format(**locals()))
+            flash("{count} dag runs were set to running".format(**locals()))
         except Exception as ex:
-            flash(str(ex))
+            flash(str(ex), 'error')
             flash('Failed to set state', 'error')
         return redirect(self.route_base + '/list')