settings: Rename 'night mode' to 'dark theme'.

Fixes: zulip#5169
Changed night mode to dark theme. Added tests, migrations, and updated comments.

Author: aritroCoder

docs/changelog.md

@@ -1900,7 +1900,7 @@ Bugfixes and other improvements for your Zulip experience.
 ### Highlights for users (since 26.1.124 / 26.2.125)
 
 * Highlight colors for code blocks now match the webapp and
-  offer more contrast, especially in Dark theme.
+  offer more contrast, especially in night mode.
 
 Plus, like every release, many other improvements for your Zulip
 experience.
@@ -1919,7 +1919,7 @@ experience.
 ### Highlights for users (since 26.1.124 / 26.2.125)
 
 * Highlight colors for code blocks now match the webapp and
-  offer more contrast, especially in Dark theme.
+  offer more contrast, especially in night mode.
 
 Plus, like every release, many other improvements for your Zulip
 experience.

static/translations/messages_tl.json

@@ -156,13 +156,8 @@
   "{realm} is running Zulip Server {serverVersion}, which is unsupported. Please contact your administrator about upgrading.": "{realm} is running Zulip Server {serverVersion}, which is unsupported. Please contact your administrator about upgrading.",
   "Learn more": "Learn more",
   "Full profile": "Full profile",
-<<<<<<< HEAD
-  "Settings": "Settings",
-  "Night mode": "Night mode",
-=======
   "Settings": "Mga Itinakda",
   "Night mode": "Night mode",
->>>>>>> c8af6d69ecbed660643facc1e84b416e3dbff879
   "Open links with in-app browser": "Open links with in-app browser",
   "Language": "Wika",
   "Arabic": "Arabic",