✨ Add new connector

[tjarbo]

As a User I want to add a new connector so that I can notify my friends on different platforms

Description

With this PR it is possible to add further connectors! Unfortunately, right now only from type discord but hopefully more are coming soon. Please have look for typos, missing translations, etc but also for security relevant topics! Maybe you have even time to try out the web interface :innocent:

Thank you!

PS: Sorry for those many changes, the PR covers some bug fixed too - remaining from the last PRs :roll_eyes:

Screenshots

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Checklist

• [ ] ~Make sure you are requesting to main. Don't request to release branch!~
• [X] I have linked the related issue.
• [X] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [X] I have added tests to cover my changes.
• [X] All new and existing tests passed.

[tjarbo]

Thank you very much for your review @p-fruck! @antonplagemann or @piuswalter: Can any of you look over the code as well this week?