Add "Load more" button to environment dropdown

What does this MR do and why?

Add "Load more" button to environment dropdown

• previously the dropdown would load all environments
• limit the number of environments loaded and add a button to explicitely load more
• consolidate Vuex logic into one file
• create new Vuex state to track environments status
• update tests (I did get a little carried away with updating the tests, but they all look a lot better now)

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Navigate to a project with Security & Compliance enabled
3. Create some environments in Infastructure => Environments
4. Navigate to Security & Compliage => Policies
5. Verify the Environment filter
6. Click the New Policy button
7. Verify the Environment filter on the Policy Editor page
8. Navigate to Security & Compliage => Threat Monitoring => Statistics tab
9. Use this snippet to create some data
10. Verify the Environment filter on the Policy Editor page

Screenshots or screen recordings

Page	Screenshot
Policies List - All Environments option
Policies List - Load more button
Changing Environments
Loading
Policies List - when there are no more environments to load
Policy Editor
Statistics Tab

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 #340339 (closed)