Add placeholder git panel (#21894)

Adds a simple git placeholder panel for us to iterate from. Also
includes a number of assets from the git prototyping branch that we will
use.

Note: This panel is staff flagged for now.

Release Notes:

- N/A

crates/feature_flags/src/feature_flags.rs

@@ -64,6 +64,11 @@ impl FeatureFlag for ZetaFeatureFlag {
     const NAME: &'static str = "zeta";
 }
 
+pub struct GitUiFeatureFlag;
+impl FeatureFlag for GitUiFeatureFlag {
+    const NAME: &'static str = "git-ui";
+}
+
 pub struct Remoting {}
 impl FeatureFlag for Remoting {
     const NAME: &'static str = "remoting";