Followers-only post federation (#2111)

* Make private toots get PuSHed to subscription URLs that belong to domains where you have approved followers

* Authorized followers controller, stub for bulk action

* Soft block in the background

* Add simple test for new controller

* Rename Settings::FollowersController to Settings::FollowerDomainsController, paginate results,
rename "private" post setting to "followers-only", fix pagination style, improve post privacy
preferences style, improve warning style

* Extract compose form warnings into own container, show warning when posting to followers-only with unlocked account

app/assets/javascripts/components/features/compose/components/warning.jsx

@@ -0,0 +1,25 @@
+import PropTypes from 'prop-types';
+
+class Warning extends React.PureComponent {
+
+  constructor (props) {
+    super(props);
+  }
+
+  render () {
+    const { message } = this.props;
+
+    return (
+      <div className='compose-form__warning'>
+        {message}
+      </div>
+    );
+  }
+
+}
+
+Warning.propTypes = {
+  message: PropTypes.node.isRequired
+};
+
+export default Warning;