Enable sanity checks for PR approval

[Kobzol]

Homu does some checks that prevent it to approve PRs that contain some messages in the title (e.g. [do not merge] etc.). Bors should do the same. Example here.

• Draft status (Disallow approving non open PRs #336)
• WIP title (Do not allow approving WIP PRs #366)
• Blocking labels (Forbid approving PRs with labels blocking approval #367)

[jieyouxu]

(Apparently homu doesn't check draft, cf. #333)

[Kobzol]

Another example: reject approvals if certain labels are set (#t-lang/meetings > Triage meeting 2025-07-23 @ 💬).