Bring concerns visual to modern GitHub UI feature

[Urgau]

@jieyouxu and I had a long and productive discussions about the visual of concerns, and in particular the fact that they can be buried in a wall of text, see rust-lang/compiler-team#864 (comment) as an example.

We therefor iterated to multiple design and ideas, and we end-up makes 3 choices to drastically improve the visual:

1. the all section is placed inside a [!CAUTION] or [!NOTE]
2. the number of active concerns is put next to the title, ie. Concerns (3 active)
3. we removed the author from the display
   • as it doesn't seems to bring much and can be detrimental to the readability, particularly when a concern is resolved (can still be fetch by just clicking on the issue comment)

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Caution

Concerns (1 active)

• This is my concern about concern
• This is a resolved concern resolved in this comment

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Note

Concerns (0 active)

• This is a resolved concern resolved in this comment

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