[MODERATION] add user blocking API

- Follow up for: #540, #802
- Add API routes for user blocking from user and organization
perspective.
- The new routes have integration testing.
- The new model functions have unit tests.
- Actually quite boring to write and to read this pull request.

(cherry picked from commit f3afaf15c7)

routers/web/user/setting/blocked_users.go

@@ -6,6 +6,7 @@ package setting
 import (
 	"net/http"
 
+	"code.gitea.io/gitea/models/db"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
@@ -23,7 +24,7 @@ func BlockedUsers(ctx *context.Context) {
 	ctx.Data["BaseLink"] = setting.AppSubURL + "/user/settings/blocked_users"
 	ctx.Data["BaseLinkNew"] = setting.AppSubURL + "/user/settings/blocked_users"
 
-	blockedUsers, err := user_model.ListBlockedUsers(ctx, ctx.Doer.ID)
+	blockedUsers, err := user_model.ListBlockedUsers(ctx, ctx.Doer.ID, db.ListOptions{})
 	if err != nil {
 		ctx.ServerError("ListBlockedUsers", err)
 		return