FE: Implement infinite scrolling for alert list

Why are we doing this work

• when a user is viewing alerts, they would not want to see too many at once
• allow for a user to see 20 alerts at a time and at the bottom of the page more are loaded

Design

• https://www.youtube.com/watch?v=HuZgaMKdJF0&feature=youtu.be

Non-functional requirements

• Documentation: I am waiting until the end to update the documentation as this feature is behind a feature flag
• Feature flag: This feature is behind the threat_monitoring_alerts feature flag
• Testing

Implementation plan

• frontend implement infinite scrolling like in https://gitlab.com/gitlab-org/gitlab/blob/master/ee%2Fapp%2Fassets%2Fjavascripts%2Fsecurity_dashboard%2Fcomponents%2Fproject_vulnerabilities.vue