Add message deletion endpoint

[Era-Dorta]

This PR adds the end point to delete messages. I've tested this both in normal and in the json-rpc modes.

This closes #558

[bbernhard]

Many thanks for the contribution!

I quickly scrolled through your PR and it looks good so far. I'll have a more detailed look on the weekend, when I have a bit more time :)

[bbernhard]

Thanks for the contribution and sorry for the late reply!

In general it looks really good, the only small suggestion I'd have is:

Instead of checking with strings.HasPrefix whether it is a group, please use the newly introduced function getRecipientType for that. The big advantage of that one is, that it not only checks if it is a phone number or a group, but also if it is a username.

[Era-Dorta]

Thanks for the review. I've fixed it in de2cb04.

I checked that it still works both with usernames and with group ids.

Is there anything that I need to do to get the CI to pass? It looks like an error unrelated to any of the code changes that I've done.

[bbernhard]

I think the CI failures are because the CI pipeline isn't properly configured to handle pull requests. Need to have a look at that one at some point. But for now, I'll just merge the changes.

Thanks!