Dropzone styling improvements (#15291)

* Dropzone styling improvements

- Move all dropzone styles to separate file
- Fix white background in arc-green
- Fix rendering of non-square images and previews

* increase thumbnail quality, set contain in js, replace blur effect with opacity

web_src/less/index.less

@@ -2,8 +2,9 @@
 
 @import "./variables.less";
 @import "./shared/issuelist.less";
-@import "./features/gitgraph.less";
 @import "./features/animations.less";
+@import "./features/dropzone.less";
+@import "./features/gitgraph.less";
 @import "./features/heatmap.less";
 @import "./features/imagediff.less";
 @import "./features/codeeditor.less";