Clearer labelling for custom value stream dropdown

What does this MR do and why?

This MR makes some UX changes to properly separate the custom value stream selector. This MR:

• Moves the dropdown and aggregation status to a new line
• Moves the dropdown to the left side
• Cleans up some margins between components on the page

Screenshots

Before	After

How to set up and validate locally

1. Ensure you have an EE license, or stub the cycle_analytics_for_groups license feature to true
2. Create a group
3. (optional) Create a project within the group (make sure to initialize the project with a readme, or create it from a template)
4. Navigate to value stream analytics for the group, ie for a group name new-vsa-group it would be http://<gdk>/group/new-vsa-group/-/analytics/value_stream_analytics (or you can navigate from the left menu)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #349677 (closed)