settings: Add automatic theme detection feature.

With this implementation of the feature of the automatic theme
detection, we make the following changes in the backend, frontend and
documentation.

This replaces the previous night_mode boolean with an enum, with the
default value being to use the prefers-color-scheme feature of the
operating system to determine which theme to use.

Fixes: #14451.

Co-authored-by: @kPerikou <44238834+kPerikou@users.noreply.github.com>

zerver/migrations/0290_remove_night_mode_add_color_scheme.py

@@ -0,0 +1,35 @@
+# Generated by Django 2.2.13 on 2020-06-20 15:22
+
+from django.db import migrations, models
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+COLOR_SCHEME_AUTOMATIC = 1
+COLOR_SCHEME_NIGHT = 2
+
+# Set color_scheme to night mode, if night_mode is True.
+def set_color_scheme_to_night_mode(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    UserProfile = apps.get_model("zerver", "UserProfile")
+    UserProfile.objects.filter(night_mode=True).update(color_scheme=COLOR_SCHEME_NIGHT)
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0289_tighten_attachment_size'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='userprofile',
+            name='color_scheme',
+            field=models.PositiveSmallIntegerField(default=COLOR_SCHEME_AUTOMATIC),
+        ),
+        migrations.RunPython(
+            set_color_scheme_to_night_mode,
+            reverse_code=migrations.RunPython.noop,
+            elidable=True),
+        migrations.RemoveField(
+            model_name='userprofile',
+            name='night_mode',
+        ),
+    ]