Alerting: Add RBAC logic for silences creation

[tomratcliffe]

What is this feature?

Adds RBAC logic for Silences creation UI (WIP)

[tomratcliffe]

/deploy-to-hg

[ephemeral-instances-bot[bot]]

• Preparing your instance. A comment containing your instance's url will be added to this PR when the instance is ready.
• Your instance will be ready in ~10 minutes. Follow the workflow progress
• Slack channel: #proj-ephemeral-hg-instances
• Building instance with alerting/silences-rbac-ui oss branch and main enterprise branch. How to choose a branch

[ephemeral-instances-bot[bot]]

• Your instance can be accessed at: https://ephemeral1511182187322tomratc.grafana-dev.net
• The instance is not using the CDN assets.
• How to access / How to update instance config / How to build a specific branch

[tomratcliffe]

/deploy-to-hg

[ephemeral-instances-bot[bot]]

• Preparing your instance. A comment containing your instance's url will be added to this PR when the instance is ready.
• Your instance will be ready in ~10 minutes. Follow the workflow progress
• Slack channel: #proj-ephemeral-hg-instances
• Building instance with alerting/silences-rbac-ui oss branch and main enterprise branch. How to choose a branch

[ephemeral-instances-bot[bot]]

Error building instance: Contact #proj-ephemeral-hg-instances if it is not a compile error. Logs

Error message

handling pull request comment event: calling gcom to upsert instance: fetching instance by slug: unexpected response status: status=502 responseBody=

Error: Server Error

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