topics: Add topic list page

For the topic-list page app bar, we leave out the icon "chveron_down.svg"
since it's related to a new design (#1039) we haven't implemented yet.
This also why "TOPICS" is not aligned to the middle part of the app bar
on the message-list page.

We also leave out the new topic button and topic filtering, which are
out-of-scope for #1158.

The topic-list implementation is quite similar to parts of inbox page
and message-list page.  Therefore, we structure the code to make them
look similar to compare for changes side-by-side to help with reviewing
what has changed.

Figma design:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6819-35869&m=dev

Author: PIG208

assets/l10n/app_en.arb

@@ -769,6 +769,10 @@
   "@mainMenuMyProfile": {
     "description": "Label for main-menu button leading to the user's own profile."
   },
+  "topicsButtonLabel": "TOPICS",
+  "@topicsButtonLabel": {
+    "description": "Label for message list button leading to topic-list page. (Use ALL CAPS for cased alphabets: Latin, Greek, Cyrillic, etc.)"
+  },
   "channelFeedButtonTooltip": "Channel feed",
   "@channelFeedButtonTooltip": {
     "description": "Tooltip for button to navigate to a given channel's feed"

lib/generated/l10n/zulip_localizations.dart

@@ -1151,6 +1151,12 @@ abstract class ZulipLocalizations {
   /// **'My profile'**
   String get mainMenuMyProfile;
 
+  /// Label for message list button leading to topic-list page. (Use ALL CAPS for cased alphabets: Latin, Greek, Cyrillic, etc.)
+  ///
+  /// In en, this message translates to:
+  /// **'TOPICS'**
+  String get topicsButtonLabel;
+
   /// Tooltip for button to navigate to a given channel's feed
   ///
   /// In en, this message translates to:

lib/generated/l10n/zulip_localizations_ar.dart

@@ -629,6 +629,9 @@ class ZulipLocalizationsAr extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_de.dart

@@ -629,6 +629,9 @@ class ZulipLocalizationsDe extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_en.dart

@@ -629,6 +629,9 @@ class ZulipLocalizationsEn extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_ja.dart

@@ -629,6 +629,9 @@ class ZulipLocalizationsJa extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_nb.dart

@@ -629,6 +629,9 @@ class ZulipLocalizationsNb extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'My profile';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';
 

lib/generated/l10n/zulip_localizations_pl.dart

@@ -638,6 +638,9 @@ class ZulipLocalizationsPl extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'Mój profil';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Strumień kanału';
 

lib/generated/l10n/zulip_localizations_ru.dart

@@ -642,6 +642,9 @@ class ZulipLocalizationsRu extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'Мой профиль';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Лента канала';
 

lib/generated/l10n/zulip_localizations_sk.dart

@@ -631,6 +631,9 @@ class ZulipLocalizationsSk extends ZulipLocalizations {
   @override
   String get mainMenuMyProfile => 'Môj profil';
 
+  @override
+  String get topicsButtonLabel => 'TOPICS';
+
   @override
   String get channelFeedButtonTooltip => 'Channel feed';