VSA Let users know data is aggregating

What does this MR do and why?

With the new VSA data aggregation backend, there can be a delay before data has been aggregated ready for display. This MR adds an alert informing user's about the possible delay in data populating, with a button to refresh the page.

This MR:

• displays the alert when a new value stream is created
• displays the alert when the last_aggregated_at timestamp is null

Screenshots or screen recordings

Directly after creating a value stream	Aggregation is ready
aggregation-warning-720	Screen_Recording_2022-04-12_at_11.54.03_am

How to set up and validate locally

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

1. Enable the vsa data aggregation feature flag

   Feature.enable(:use_vsa_aggregated_tables)

2. Ensure you have an EE license, or stub the cycle_analytics_for_groups license feature to true

3. Create a brand new group

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)

5. You will see the empty state, with a link to the "create value stream" form

6. Create a value stream, the "Data is aggregating" banner should now display

7. Click "Reload page", the data should still be aggregating

8. Wait for 10mins - Data aggregation takes at least ~10mins to complete for a new group

9. Refresh the page, the value new value stream should be available

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 #352427 (closed)