edit predictions: Enable blog post link behind a feature flag (#24720)

This PR updates the blog post link in the onboarding modal to be behind
the `predict-edits-launch` feature flag instead of a staff flag.

This will allow us to enable the blog post link once we're live.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/feature_flags/src/feature_flags.rs

@@ -64,6 +64,12 @@ impl FeatureFlag for PredictEditsFeatureFlag {
     const NAME: &'static str = "predict-edits";
 }
 
+/// A feature flag that controls things that shouldn't go live until the predictive edits launch.
+pub struct PredictEditsLaunchFeatureFlag;
+impl FeatureFlag for PredictEditsLaunchFeatureFlag {
+    const NAME: &'static str = "predict-edits-launch";
+}
+
 pub struct PredictEditsRateCompletionsFeatureFlag;
 impl FeatureFlag for PredictEditsRateCompletionsFeatureFlag {
     const NAME: &'static str = "predict-edits-rate-completions";