Use smaller notices on notices page

[samhotep]

Note

• Should only be reviewed after https://github.com/canonical/ubuntu-com-security-api/pull/163 has been merged

Done

• Load notices on the /security/notices page from a new datasource get_page_notice
• This returns a smaller notice, without the cves and packages fields which are not required on this page, and have a significant effect on page speed, especially when a notice has hundreds of cves.

QA

• Check that the demo runs
• Open demo_url/security/notices and click through the pagination
• Pages should open quickly without timeouts