subscription_list: Change channelsEmptyPlaceholder to use "You're"

To be a bit less stuffy:
  zulip#1849 (comment)

Author: chrisbobbe

assets/l10n/app_en.arb

@@ -1074,7 +1074,7 @@
   "@channelsPageTitle": {
     "description": "Title for the page with a list of subscribed channels."
   },
-  "channelsEmptyPlaceholder": "You are not subscribed to any channels yet.",
+  "channelsEmptyPlaceholder": "You’re not subscribed to any channels yet.",
   "@channelsEmptyPlaceholder": {
     "description": "Centered text on the 'Channels' page saying that there is no content to show."
   },

lib/generated/l10n/zulip_localizations.dart

@@ -1588,7 +1588,7 @@ abstract class ZulipLocalizations {
   /// Centered text on the 'Channels' page saying that there is no content to show.
   ///
   /// In en, this message translates to:
-  /// **'You are not subscribed to any channels yet.'**
+  /// **'You’re not subscribed to any channels yet.'**
   String get channelsEmptyPlaceholder;
 
   /// Title for the page about sharing content received from other apps.

lib/generated/l10n/zulip_localizations_ar.dart

@@ -898,7 +898,7 @@ class ZulipLocalizationsAr extends ZulipLocalizations {
 
   @override
   String get channelsEmptyPlaceholder =>
-      'You are not subscribed to any channels yet.';
+      'You’re not subscribed to any channels yet.';
 
   @override
   String get sharePageTitle => 'Share';

lib/generated/l10n/zulip_localizations_en.dart

@@ -898,7 +898,7 @@ class ZulipLocalizationsEn extends ZulipLocalizations {
 
   @override
   String get channelsEmptyPlaceholder =>
-      'You are not subscribed to any channels yet.';
+      'You’re not subscribed to any channels yet.';
 
   @override
   String get sharePageTitle => 'Share';

lib/generated/l10n/zulip_localizations_fr.dart

@@ -912,7 +912,7 @@ class ZulipLocalizationsFr extends ZulipLocalizations {
 
   @override
   String get channelsEmptyPlaceholder =>
-      'You are not subscribed to any channels yet.';
+      'You’re not subscribed to any channels yet.';
 
   @override
   String get sharePageTitle => 'Share';

lib/generated/l10n/zulip_localizations_nb.dart

@@ -898,7 +898,7 @@ class ZulipLocalizationsNb extends ZulipLocalizations {
 
   @override
   String get channelsEmptyPlaceholder =>
-      'You are not subscribed to any channels yet.';
+      'You’re not subscribed to any channels yet.';
 
   @override
   String get sharePageTitle => 'Share';

lib/generated/l10n/zulip_localizations_sk.dart

@@ -900,7 +900,7 @@ class ZulipLocalizationsSk extends ZulipLocalizations {
 
   @override
   String get channelsEmptyPlaceholder =>
-      'You are not subscribed to any channels yet.';
+      'You’re not subscribed to any channels yet.';
 
   @override
   String get sharePageTitle => 'Share';

lib/generated/l10n/zulip_localizations_zh.dart

@@ -898,7 +898,7 @@ class ZulipLocalizationsZh extends ZulipLocalizations {
 
   @override
   String get channelsEmptyPlaceholder =>
-      'You are not subscribed to any channels yet.';
+      'You’re not subscribed to any channels yet.';
 
   @override
   String get sharePageTitle => 'Share';

test/widgets/subscription_list_test.dart

@@ -62,7 +62,7 @@ void main() {
     check(getItemCount()).equals(0);
     check(isPinnedHeaderInTree()).isFalse();
     check(isUnpinnedHeaderInTree()).isFalse();
-    check(find.text('You are not subscribed to any channels yet.')).findsOne();
+    check(find.text('You’re not subscribed to any channels yet.')).findsOne();
   });
 
   testWidgets('basic subscriptions', (tester) async {