Make theme directory and Rename application.scss (#5336)

* Rename application.scss

* Move to 'default' directory

* Follow review

Move directory, and Rename "mastodon".

* Revert rename

* undo removal of newline

app/javascript/styles/application.scss

-@import 'mixins';
-@import 'variables';
+@import 'mastodon/mixins';
+@import 'mastodon/variables';
 @import 'fonts/roboto';
 @import 'fonts/roboto-mono';
 @import 'fonts/montserrat';
 
-@import 'reset';
-@import 'basics';
-@import 'containers';
-@import 'lists';
-@import 'footer';
-@import 'compact_header';
-@import 'landing_strip';
-@import 'forms';
-@import 'accounts';
-@import 'stream_entries';
-@import 'components';
-@import 'emoji_picker';
-@import 'about';
-@import 'tables';
-@import 'admin';
-@import 'rtl';
+@import 'mastodon/reset';
+@import 'mastodon/basics';
+@import 'mastodon/containers';
+@import 'mastodon/lists';
+@import 'mastodon/footer';
+@import 'mastodon/compact_header';
+@import 'mastodon/landing_strip';
+@import 'mastodon/forms';
+@import 'mastodon/accounts';
+@import 'mastodon/stream_entries';
+@import 'mastodon/components';
+@import 'mastodon/emoji_picker';
+@import 'mastodon/about';
+@import 'mastodon/tables';
+@import 'mastodon/admin';
+@import 'mastodon/rtl';