New notification provider: Threema Gateway

[booooza]

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Description

Adds a new notification provider for Threema Gateway

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generates no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots

[booooza]

@threema-eduard fyi

[CommanderStorm]

Nice work. Looks basically mergable with a few minor changes

I have left a few minor comments below.

[booooza]

@CommanderStorm Thanks for the fast review! I've included the updated changes. 👍

[CommanderStorm]

Thanks for the new notification provider! 🎉

Note

This PR is part of the v2.0 merge window => see #4500 for the bugs that need to be addressed before we can ship this release ^^

_{All changes in this PR are small and uncontroversial ⇒ merging with junior maintainer approval}