feat(push): Add option to send a self-test

[nickvergessen]

Fix #1206

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[Ivansss]

Tested and work 👍

[SystemKeeper]

Should we rate limit that endpoint?

[miaulalala]

Nice! But I agree with @SystemKeeper

[nickvergessen]

Had discussed earlier already, that there are many other points we would need to ratelimit to avoid a problem here (e.g. create a public conversation and mention yourself, create a public share, set activity to push on upload and bombard yourself).

And at the same time, we might want to have a temporary high frequency here anyway. I would only imagine something like 60 per hour?