Environments Dashboard Limits and Optimization

What does this MR do?

• Impose limits on the Environments Dashboard. The dashboard will only display up to 7 projects and 3 environments per project. These seem like reasonable limits based on investigation into current Operations Dashboard usage: https://gitlab.com/gitlab-org/gitlab/issues/34945
• Avoid exposing unnecessary entities in order to reduce the computation time and queries.
• Preload some models to reduce the number of database queries.

Issue: #3713 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team