Fetch private repository milestones ids instead of manually harcoding

[ksy36]

We're currently doing that for public repo:

https://github.com/webcompat/webcompat.com/blob/12031a8d5396778418ba941a8534cc3dd30ff33c/config/init.py#L54-L72

We need to eliminate hardcoded milestones ids for the private repo, in case those ids change at some point in time.