Alert Preview Feature for GraphQL API and Create Alert Dialog

[mastercactapus]

What problem would you like to solve? Please describe: Users need a method to visualise how their configured alerts would look when sent through different platforms (like Slack, SMS, etc.) The current practice of sending a test alert can be disruptive, especially when a service is already setup with on-call rotations, and a user is working to expand monitoring.

Describe the solution you'd like: Implement an alert "preview" feature primarily as part of the GraphQL API, which would be particularly useful with the introduction of the signal work where additional rendering comes into play.

In the Create Alert Dialog, incorporate a "preview" tab on the 'details' section (similar to GitHub) for users to see how their markdown text would render.

Additional context: This feature would provide a friendly and non-disruptive means for users to see the whitespace-removed and rendered version of their alerts before they are sent. It would prove especially useful during the setup phase of monitoring and could potentially benefit other systems integrating into GoAlert or GoAlert itself.