Add mattermost notifications

[nbrinckm]

We want to get the notifications also in the riegsgos mattermost:

https://mattermost.hzdr.de/riesgos-2-dev/channels/github-notifications

[nbrinckm]

webhook url is in the mattermost

https://mattermost.hzdr.de/riesgos-2-dev/pl/ztgm7zgb5pypfdnzpjetpih4yo

[nbrinckm]

Direct webhook integration for github webhooks doesn't seem to work (doesn't have a text field that the mattermost would need).

[nbrinckm]

But @MichaelLangbein found an github action:
https://github.com/mattermost/action-mattermost-notify

[nbrinckm]

@MichaelLangbein Does you already have some experience in using github actions? I haven't honestly.

[MichaelLangbein]

@MichaelLangbein Does you already have some experience in using github actions? I haven't honestly.

Unfortunately not! I wouldn't mind doing some research on this - but probably only have time after the current sprint.

[nbrinckm]

@MichaelLangbein Does you already have some experience in using github actions? I haven't honestly.

Unfortunately not! I wouldn't mind doing some research on this - but probably only have time after the current sprint.

That is ok I think