Editor default options refactoring + include imageUploader plugin

In order to use imageUploader plugin:
- js/css files must included in the app;
- an upload method must be provided in Active Admin entity;
- the image_uploader server_url field option needs to be set.

app/assets/stylesheets/activeadmin/quill.imageUploader.min.css

@@ -0,0 +1,33 @@
+.image-uploading {
+  position: relative;
+  display: inline-block;
+}
+
+.image-uploading img {
+  max-width: 98% !important;
+  filter: blur(5px);
+  opacity: 0.3;
+}
+
+.image-uploading::before {
+  content: "";
+  box-sizing: border-box;
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  width: 30px;
+  height: 30px;
+  margin-top: -15px;
+  margin-left: -15px;
+  border-radius: 50%;
+  border: 3px solid #ccc;
+  border-top-color: #1e986c;
+  z-index: 1;
+  animation: spinner 0.6s linear infinite;
+}
+
+@keyframes spinner {
+  to {
+    transform: rotate(360deg);
+  }
+}