Add SLA panel with customizable weights

Add SLA panel with customizable weights

This adds an extra panel and templates to the general SLA dashboard.

The templates allow selecting a weight per service, and the small SLA panel will display the overall system availability with the weights selected in the templates.

There might be a small drift between the values from the source metric (slo_observation_status) and the recorded SLA. These are rounding errors that should not exceed 0.01%, but they could cause the panels to colour differently.

Snapshot: https://dashboards.gitlab.net/dashboard/snapshot/JdNv4Xu4TSYfUFSFtbbkyrSUQIVAAggj?orgId=1&var-sidekiq_weight=1

For: gitlab-com/gl-infra/scalability#2370 (closed)