Add Migadu provider

[achou11]

I was surprised to see that https://migadu.com is not on the list. I use Delta Chat (on Android + Desktop) with my Migadu account and I haven't had any major issues. There are daily receiving and sending limits based on the plan you subscribe to but I haven't reached them while using Delta Chat (I'm on their mini plan which allows 1000 receiving and 100 sending per day)

Happy to create a PR to add details for the database when I find time :)

[achou11]

ah just saw the open PR for Migadu. can close this issue I guess 😄