Webmention support
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## Description including problem, use cases, benefits, and/or goals
The web is amazing, and it became amazing because of Hypertext.
Imagine you create an issue on your GitLab, mentioning an issue from my GitLab.
Your GitLab could understand that the link points to another GitLab issue, and decide to webmention it!
Then we have a federated issue tracker!
It is actually already working inside github, but the idea would be to have it federated between all the GitLab instances of the world (and maybe github, we can dream)
## Proposal
Implement webmention on the issue tracker
## Links / references
http://webmention.net/
~"feature proposal"
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