tag: re-add {{dead end}}, add {{sources exist}} per requests

[siddharthvp]

Now that the interface supports search, clutter is no longer a reason for removing templates.
Adding back {{dead end}} was requested here. Also adding {{sources exist}}, requested here

[Amorymeltzer]

We should probably put together some sort of criteria for inclusion; less important than #533 perhaps, but thinking of #178.

[MusikAnimal]

We should probably put together some sort of criteria for inclusion; less important than #533 perhaps, but thinking of #178.

I agree. In this case {{dead end}} has sound rationale for inclusion, and {{sources exits}} had rough consensus. My concerns were alleviated with the new search feature, but if I was out of line in merging this I do apologize. Some sort of guideline would be helpful, noting that transclusion count doesn't always equate to usage, as is the case with short-lived tags like {{dead end}}.

[Amorymeltzer]

Not at all @MusikAnimal! Just noting that we've now moved from needing to be actively limited in our offerings to being able to be more inclusive. I do think we should be cognizant of the position Twinkle is in — if something is listed, it's likely to get used more — but this is entirely in order. We would want a way to assess items that are quickly removed, as the scripts in #178 (comment) and #178 (comment) obviously won't help.