Add missing notifiers to scalingo_alert

[mickaelalibert]

Hello, this PR is a small improvement suggestion :

The scalingo_alert table doesn't contain the notifiers id list, but this information is available via the Scalingo API.

Even though the documentation states that the notifiers information is only available while listing all alerts, I found out that the scalingo client also returns the notifiers while fetching a specific alert via its id.

[mickaelalibert]

Seems that the reason why I was able to fetch notifiers in a specific scalingo_alert get is that the result was cached from my previous list query.
So the scalingo client is doing exactly as the documentation states, and it is not yet possible to fetch notifiers info on a specific alert. 😞

[francois2metz]

I'm keeping it open, maybe Scalingo will return the notifiers on a single result.

[mickaelalibert]

Scalingo confirmed that they are working on it as this was not the expected functionality.
I'll keep this PR updated when Scalingo applies the patch to its API.