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_update.go

@@ -517,6 +517,15 @@ func FindAndUpdateIssueMentions(ctx context.Context, issue *Issue, doer *user_mo
 	if err != nil {
 		return nil, fmt.Errorf("UpdateIssueMentions [%d]: %w", issue.ID, err)
 	}
+
+	notBlocked := make([]*user_model.User, 0, len(mentions))
+	for _, user := range mentions {
+		if !user_model.IsUserBlockedBy(ctx, doer, user.ID) {
+			notBlocked = append(notBlocked, user)
+		}
+	}
+	mentions = notBlocked
+
 	if err = UpdateIssueMentions(ctx, issue.ID, mentions); err != nil {
 		return nil, fmt.Errorf("UpdateIssueMentions [%d]: %w", issue.ID, err)
 	}