Add user blocking (#29028)

Fixes #17453

This PR adds the abbility to block a user from a personal account or
organization to restrict how the blocked user can interact with the
blocker. The docs explain what's the consequence of blocking a user.

Screenshots:


![grafik](4ed884f3-e06a-4862-afd3-3b8aa2488dc6)


![grafik](ae6d4981-f252-4f50-a429-04f0f9f1cdf1)


![grafik](ca153599-5b0f-4b4a-90fe-18bdfd6f0b6b)

---------

Co-authored-by: Lauris BH <lauris@nix.lv>

models/issues/issue_xref.go

@@ -214,6 +214,10 @@ func (issue *Issue) verifyReferencedIssue(stdCtx context.Context, ctx *crossRefe
 		if !perm.CanReadIssuesOrPulls(refIssue.IsPull) {
 			return nil, references.XRefActionNone, nil
 		}
+		if user_model.IsUserBlockedBy(stdCtx, ctx.Doer, refIssue.PosterID, refIssue.Repo.OwnerID) {
+			return nil, references.XRefActionNone, nil
+		}
+
 		// Accept close/reopening actions only if the poster is able to close the
 		// referenced issue manually at this moment. The only exception is
 		// the poster of a new PR referencing an issue on the same repo: then the merger