Use options rather than a boolean

This prepares for a third option that would restrict privacy to
privacyPreference(replyPrivacy, sideArmBasePrivacy)

app/javascript/flavours/glitch/features/composer/index.js

@@ -51,6 +51,9 @@ import { wrap } from 'flavours/glitch/util/redux_helpers';
 //  State mapping.
 function mapStateToProps (state) {
   const inReplyTo = state.getIn(['compose', 'in_reply_to']);
+  const replyPrivacy = inReplyTo ? state.getIn(['statuses', inReplyTo, 'visibility']) : null;
+  const sideArmBasePrivacy = state.getIn(['local_settings', 'side_arm']);
+  const sideArmPrivacy = (state.getIn(['local_settings', 'side_arm_reply_mode']) === 'copy' ? replyPrivacy : null) || sideArmBasePrivacy;
   return {
     acceptContentTypes: state.getIn(['media_attachments', 'accept_content_types']).toArray().join(','),
     advancedOptions: state.getIn(['compose', 'advanced_options']),
@@ -67,7 +70,7 @@ function mapStateToProps (state) {
     replyAccount: inReplyTo ? state.getIn(['statuses', inReplyTo, 'account']) : null,
     replyContent: inReplyTo ? state.getIn(['statuses', inReplyTo, 'contentHtml']) : null,
     resetFileKey: state.getIn(['compose', 'resetFileKey']),
-    sideArm: (state.getIn(['local_settings', 'side_arm_auto']) ? state.getIn(['compose', 'reply_privacy']) : null) || state.getIn(['local_settings', 'side_arm']),
+    sideArm: sideArmPrivacy,
     sensitive: state.getIn(['compose', 'sensitive']),
     showSearch: state.getIn(['search', 'submitted']) && !state.getIn(['search', 'hidden']),
     spoiler: state.getIn(['compose', 'spoiler']),

app/javascript/flavours/glitch/features/local_settings/page/index.js

@@ -14,6 +14,8 @@ const messages = defineMessages({
   layout_desktop: { id: 'layout.desktop', defaultMessage: 'Desktop' },
   layout_mobile: { id: 'layout.single', defaultMessage: 'Mobile' },
   side_arm_none: { id: 'settings.side_arm.none', defaultMessage: 'None' },
+  side_arm_keep: { id: 'settings.side_arm_reply_mode.keep', defaultMessage: 'Keep secondary toot button to set privacy' },
+  side_arm_copy: { id: 'settings.side_arm_reply_mode.copy', defaultMessage: 'Copy privacy setting of the toot being replied to' },
 });
 
 @injectIntl
@@ -80,6 +82,10 @@ export default class LocalSettingsPage extends React.PureComponent {
             settings={settings}
             item={['side_arm_reply_mode']}
             id='mastodon-settings--side_arm_reply_mode'
+            options={[
+              { value: 'keep', message: intl.formatMessage(messages.side_arm_keep) },
+              { value: 'copy', message: intl.formatMessage(messages.side_arm_copy) }
+            ]}
             onChange={onChange}
           >
             <FormattedMessage id='settings.side_arm_reply_mode' defaultMessage='When replying to a toot:' />

app/javascript/flavours/glitch/reducers/local_settings.js

@@ -10,7 +10,7 @@ const initialState = ImmutableMap({
   stretch   : true,
   navbar_under : false,
   side_arm  : 'none',
-  side_arm_auto : false,
+  side_arm_reply_mode : 'keep',
   collapsed : ImmutableMap({
     enabled     : true,
     auto        : ImmutableMap({