Dropzone styling improvements (#15291) (#15374)

* 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

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_repository.less

@@ -1021,11 +1021,6 @@
             color: var(--color-text-light-2);
             font-style: italic;
           }
-
-          .dropzone-attachments .thumbnails .ui.image {
-            max-height: 150px;
-          }
-
         }
 
         .ui.form {
@@ -2417,29 +2412,6 @@
   padding-top: 15px;
 }
 
-.ui.form {
-  .dropzone {
-    border: 2px dashed var(--color-secondary);
-    background: none;
-    box-shadow: none !important;
-    padding: 0;
-    min-height: 5rem;
-    border-radius: 4px;
-
-    .dz-button {
-      color: rgba(0, 0, 0, .6);
-    }
-
-    &:hover .dz-button {
-      color: rgba(0, 0, 0, .8);
-    }
-
-    .dz-error-message {
-      top: 140px;
-    }
-  }
-}
-
 .settings {
   .content {
     margin-top: 2px;