allow CORS for api GET and OPTIONS
allow CORS for api GET and OPTIONS

GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,tinytub/alertmanager,joehandzik/alertmanager,brancz/alertmanager,joehandzik/alertmanager,tinytub/alertmanager,Kellel/alertmanager,tyrken/alertmanager,Kellel/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,brancz/alertmanager,prometheus/alertmanager,brunoqc/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,tinytub/alertmanager,brancz/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,prometheus/alertmanager,Aloomaio/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,brancz/alertmanager,brunoqc/alertmanager,joehandzik/alertmanager