[Glitch] Rewrite actions/modal and reducers/modal with typescript

Port 38c6216082 to glitch-soc

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

app/javascript/flavours/glitch/actions/onboarding.js

@@ -6,7 +6,9 @@ export function showOnboardingOnce() {
     const alreadySeen = getState().getIn(['settings', 'onboarded']);
 
     if (!alreadySeen) {
-      dispatch(openModal('ONBOARDING'));
+      dispatch(openModal({
+        modalType: 'ONBOARDING',
+      }));
       dispatch(changeSetting(['onboarded'], true));
       dispatch(saveSettings());
     }