Add dropdown for boost privacy in boost confirmation modal (#15704)

* Various dropdown code quality fixes

* Prepare support for privacy selection in boost modal

* Add dropdown for boost privacy in boost confirmation modal

app/javascript/mastodon/reducers/boosts.js

@@ -0,0 +1,25 @@
+import Immutable from 'immutable';
+
+import {
+  BOOSTS_INIT_MODAL,
+  BOOSTS_CHANGE_PRIVACY,
+} from 'mastodon/actions/boosts';
+
+const initialState = Immutable.Map({
+  new: Immutable.Map({
+    privacy: 'public',
+  }),
+});
+
+export default function mutes(state = initialState, action) {
+  switch (action.type) {
+  case BOOSTS_INIT_MODAL:
+    return state.withMutations((state) => {
+      state.setIn(['new', 'privacy'], action.privacy);
+    });
+  case BOOSTS_CHANGE_PRIVACY:
+    return state.setIn(['new', 'privacy'], action.privacy);
+  default:
+    return state;
+  }
+}