Auto merge of rust-lang#3536 - tiif:add_rustbot_feat, r=RalfJung

Add rustbot claim feature

Add rustbot ``claim``, ``release-assignment`` and ``assign-user`` as mentioned in rust-lang#3528.

rustbot issue assignment documentation: https://forge.rust-lang.org/triagebot/issue-assignment.html
pr trigger option documentation: https://forge.rust-lang.org/triagebot/pr-assignment.html#additional-new-pr-trigger-options

Author: bors

src/tools/miri/triagebot.toml

@@ -10,5 +10,10 @@ allow-unauthenticated = [
 # Gives us the commands 'ready', 'author', 'blocked'
 [shortcut]
 
+# Enables assigning users to issues and PRs.
+[assign]
+warn_non_default_branch = true
+contributing_url = "https://github.com/rust-lang/miri/blob/master/CONTRIBUTING.md"
+
 [no-merges]
 exclude_titles = ["Rustup"]