Moved actions in VSA settings pages below form

What does this MR do and why?

• Moves actions in the Value Streams settings pages from the header to the bottom of the form
• Moves headers to app.vue
• Renames value_stream_form_content_header.vue to the more descriptive value_stream_form_content_actions.vue
• Adds a Cancel action to both settings pages which, when selected, takes the user back to the VSA page (or to the selected value stream if on the Edit value stream page)
• Moves the Add a stage button to the value_stream_form_content_actions.vue component

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

	Before	After
New value stream
Edit value stream
New value stream (mobile)
Edit value stream (mobile)
Settings page form with hidden stages

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Please make sure that you're on the GitLab Ultimate plan.
2. Feature.enable(:vsa_standalone_settings_page)
3. Visit any group that has VSA data or follow these steps:
   1. run FILTER=customizable_cycle_analytics SEED_CUSTOMIZABLE_CYCLE_ANALYTICS=1 bundle exec rake db:seed_fu
   2. Visit the group's VSA. The URL is present in the output of the command above.
4. Select Edit next to the value streams dropdown and verify that actions now are below the form.
5. Select New Value Stream in the value streams dropdown and verify that actions now are below the form.

Related to #480252 (closed)