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itmpl template function

- Planned Features582
itmpl is a template function which takes a function name and an ID.
If it has already built a block with that specific ID, then it will reuse the previously built one, otherwise it will build it on the spot and cache it for potential use.

This may come in useful for things like the userinfo bits on posts, so that we can just build that bit once for a user rather than for every post they've created in a topic.