Rename `app/javascript/flavours/glitch/styles/index.scss` to `application.scss`

This is just to reduce differences with upstream.

app/javascript/flavours/glitch/styles/application.scss

@@ -8,20 +8,20 @@
 @import 'branding';
 @import 'containers';
 @import 'lists';
-@import 'modal';
 @import 'widgets';
 @import 'forms';
 @import 'accounts';
 @import 'statuses';
 @import 'components';
 @import 'polls';
+@import 'modal';
 @import 'emoji_picker';
 @import 'about';
 @import 'tables';
 @import 'admin';
-@import 'accessibility';
-@import 'rtl';
 @import 'dashboard';
+@import 'rtl';
+@import 'accessibility';
 @import 'rich_text';
 @import 'glitch_local_settings';
 @import 'glitch_doodle';

app/javascript/flavours/glitch/styles/contrast.scss

@@ -1,3 +1,3 @@
 @import 'contrast/variables';
-@import 'index';
+@import 'application';
 @import 'contrast/diff';

app/javascript/flavours/glitch/styles/mastodon-light.scss

@@ -1,3 +1,3 @@
 @import 'mastodon-light/variables';
-@import 'index';
+@import 'application';
 @import 'mastodon-light/diff';

app/javascript/skins/glitch/default/common.scss

@@ -1 +1 @@
-@import 'flavours/glitch/styles/index';
+@import 'flavours/glitch/styles/application';