Events: The priority field in the Event Definition overview is a number

## Expected Behavior
When creating an Event Definition, I can choose between multiple levels of priority (e.g., High, Low).
When seeing an Event, I also get the name of that priority shown.
When looking at the Event Definition Overview, I also expect this name to be shown.

## Current Behavior
I get shown the number, which represents the level of priority.

## Possible Solution
Map the number with the name of the level

## Your Environment
* Graylog Version: Graylog 7.0.0-SNAPSHOT+b738aa1