Support recipient-level optout for Twinkle notices

[siddharthvp]

Allows users to opt out from CSD/PROD/XFD notices by placing {{Twinkle optout}} on their talk page. The template creates an invisible external link to https://optout.twinkle with a types query param being the list of opted-out notice types. Opting out of specific CSD criteria notices is left for later, but opting out from specific XfD venues is supported. See https://en.wikipedia.org/wiki/Template:Twinkle_optout for usage details.

In Twinkle, we use a prop=extlinks API call to check for external links to that domain and parse the types parameter. An external link is used because it's the only way to create metadata that can be modified in wikitext and also accessed via the API.

This is a popular request, the most recent version being https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Mute_feature_not_working.

[NovemLinguae]

1. Is there enough consensus for this? The linked section is just one user (Dr. Blofeld) requesting it, I think.

2. The situation in the linked section was Dr. Blofeld wanted to stop JoeNMLC from leaving Twinkle messages. I don't think he said he wanted all Twinkle messages stopped. Makes me wonder if the template should support blocking notifications from a particular user as one of its params.

3. Even though you put in the docs that this shouldn't be included in templates such as {{Not around}} and {{Deceased}}, somehow I have a feeling this might eventually make its way in there, placed by someone who didn't read the docs.