Limit the maximum number of pipeline schedules per project

Problem

Currently, users can create limitless pipeline schedules per project, because of this, the feature is prone to abusers that potentially allows DOS attack, like we're currently discussing in gitlab-com/gl-infra/production#895 (comment 182148827).

Here is the number of pipeline schedules per project today. 100~300 of pipeline schedules is way too much IMO. We'd need to communicate with PM/anti-abusing team to set a reasonable borderline for the maximum number of pipeline schedules per project.

Proposal

Maybe 30 at the most?