Alerting: Remove start page of upgrade preview
Alerting upgrade page will now always show the summary table even before
upgrading any alerts or notification channels. There a few reasons for this:
- The information on the start page is redundant as it's now contained in the
documentation.
- Previously, if some unexpected issue prevented performing a full upgrade, a
user would have limited to no means to using the preview tool to help fix the
problem. This is because you could not see the summary table until the full
upgrade was performed at least once. Now, you can upgrade individual alerts and
notification channels from the beginning.
This PR has two steps that together create a functional dry-run capability for the migration.
By enabling the feature flag alertingPreviewUpgrade when on legacy alerting it will:
a. Allow all Grafana Alerting background services except for the scheduler to start (multiorg alertmanager, state manager, routes, …).
b. Allow the UI to show Grafana Alerting pages alongside legacy ones (with appropriate in-app warnings that UA is not actually running).
c. Show a new “Alerting Upgrade” page and register associated /api/v1/upgrade endpoints that will allow the user to upgrade their organization live without restart and present a summary of the upgrade in a table.
