Add a better user agent

[leahoswald]

While blocking some other bots we stumbled over SummalyBot and thought it's just another scraper, especially because not that much can be found about it. After a request, from a blocked instance, we researched more and found this repository.
I would like to suggest making the UA more verbose. Instead of just reporting as SummalyBot/$version add more info to it like SummalyBot/$version ($sourceInstanceURL; +https://github.com/misskey-dev/summaly/README.md)