FE: Create Scan Policies route and add it to the sidebar with page

Why are we doing this work

• Threat Monitoring is being updated to be more specific to the monitoring aspects (i.e. statistics, alerts, etcetera)
• Move Policies into its own section

Relevant links

Design:

• https://gitlab.com/groups/gitlab-org/-/uploads/5cc8d145388d55ac7e88233d7bdea076/policy-select.png
• https://gitlab.com/groups/gitlab-org/-/uploads/4ba546a230211450be95fc1f2808f967/policy-select-dropdown.png

Non-functional requirements

• Documentation
• Testing

Implementation plan

• frontend Create the scan policies route
• frontend Add the new scan policies route to the sidebar
• frontend Create dropdown page to select a project from gitlab that has the yaml of policies
  • A project selector is not new to the code base so there should be options to reuse some code
    • Vue component https://gitlab.com/gitlab-org/gitlab/blob/93d77316b5d03f06fd6e38f7dceb577b2df9977b/app/assets/javascripts/vue_shared/components/project_selector/project_selector.vue#L0-1
    • HAML https://gitlab.com/gitlab-org/gitlab/blob/master/app/views/clusters/clusters/_advanced_settings.html.haml#L21
• frontend save button that enables when a project changes
• Address UX issue !56356 (merged)

~~- [ ] frontend Move the policies page to that route~~ Do in &5362 (closed)