Alertmanager Secrets stored in plaintext

[jtlisi]

As things currently stand each users alertmanager configuration is stored as a plaintext JSON byte object in a postgres table. Since alertmanager configs contain api keys and passwords functionality should added to ensure the config is not stored as plaintext but instead an encrypted format of some type.

[pstibrany]

• We should document that this is the case for users to understand.
• Needs design doc first.

[friedrichg]

This bug is specific to the deprecated configs. With the new alertmanager API, secrets are stored in block-storage where the encryption is (can be) performed in the backend.