Add the concept of community reviews before PR assignment

[Urgau]

This PR adds the concept of community reviews before PR assignment.

It's enabled in the [assign] handler with a new config:

[assign.community_reviews]
minimum_approvals = 2
label = "S-waiting-on-community-reviews"

The normal workflow for community reviews is like this:

1. PR is being opened (non-draft, otherwise ignore until ready)
   • Add the label
   • Post the welcome message if new user, with small explanation of community reviews
     • No message for recurring users
2. Min approval reviews is reached (or label is manually removed)
   • Normal PR assignment
   • Assignment message

If someone is being assigned with our commands (r?, reroll), we remove the label after the assignment, thus preventing further auto-assign.

I tested what I could on my private repo, the core logic seems to work, although I couldn't test everything since I'm alone.

cc @samueltardieu

[blyxyas]

Could you make this message configurable? We'd love to link this blog post, at least temporarily, and possibly make a reviewing guide.

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[Urgau]

We already have a configurable link in the [assign] handler: contributing_url. You could change it to your blog post or link it to inside your contributing file. (The associated text is defined here)

But in general I would like to avoid making more messages configurable, it's a pain to deal with, but if the option above doesn't work let me know.

[Urgau]

Btw, feel free to suggest any text changes. I didn't think too much about them.

[Kobzol]

Thank you, this looks great, and relatively simple.

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[Kobzol]

Left one additional comment, but that's not super important I think. Feel free to merge, great work!

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