WIP: Navigate logs by cluster

What does this MR do?

• Changes the top-level navigation dropdown from environments to cluster. As we're moving to open up logs from all pods and namespaces, a cluster becomes the main unit of navigation. From the cluster depends feature such as elastic search being enabled.
• Cluster data is statically set by the backend rather than with an XHR request, reducing latency
• Instead of the logs.json api returning possible pod names next to the logs, a new logs/filters.json is created which returns all filtering information (namespaces, pods and containers) for a cluster. This reduces latency before being able to display the dropdowns, and results in better separation of responsibility in the code.
• Instead of the backend defaulting to the first pod of an environment (and first container) when none is requested, move that decision to the frontend, where it will be changed to default to "all pods" in a subsequent MR.

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