[Glitch] Rename JSX files with proper .jsx extension

Port 44a7d87cb1 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/features/generic_not_found/index.jsx

@@ -0,0 +1,11 @@
+import React from 'react';
+import Column from 'flavours/glitch/features/ui/components/column';
+import MissingIndicator from 'flavours/glitch/components/missing_indicator';
+
+const GenericNotFound = () => (
+  <Column>
+    <MissingIndicator fullPage />
+  </Column>
+);
+
+export default GenericNotFound;