Limit notices requests to 20 at most

[samhotep]

Done

• Added a hard limit to how many notices can be requested.
• This is to reduce the pressure on the backend to assemble and serve very large json payloads.

QA

• Check out this feature branch
• Run the site using the command ./run serve
• View the site locally in your web browser at: http://0.0.0.0:8030/
• Run through the following [QA steps](https://canonical-web-and-design.github
  .io/practices/workflow/qa-steps.html)
• Request more than 20 notices e.g.
• /security/notices.json?limit=21
• • Request fewer than 20 notices e.g.
• /security/notices.json?limit=19

[mtruj013]

LGTM!