js: reorganize Javascript file structure.

Move Javascript files from app/components/**/*.js back to app/javascript/src/javascripts/*.js.
This way Javascript files are in one place, which simplifies import paths and makes it
easier to see all Javascript at once.

app/javascript/src/javascripts/media_asset_component.js

@@ -0,0 +1,56 @@
+export default class MediaAssetComponent {
+  static initialize() {
+    $(".media-asset-component").toArray().forEach(element => {
+      new MediaAssetComponent(element);
+    });
+  }
+
+  constructor(element) {
+    this.$component = $(element);
+
+    if (this.$image.length) {
+      this.$image.on("click.danbooru", e => this.toggleFit());
+      new ResizeObserver(() => this.updateZoom()).observe(this.$image.get(0));
+      this.updateZoom();
+    }
+  }
+
+  toggleFit() {
+    this.$component.toggleClass("fit-screen");
+    this.updateZoom();
+  }
+
+  updateZoom() {
+    this.$image.removeClass("cursor-zoom-in cursor-zoom-out");
+    this.$zoomLevel.addClass("hidden").text(`${Math.round(100 * this.zoomLevel)}%`);
+
+    if (this.isDownscaled) {
+      this.$image.addClass("cursor-zoom-out");
+      this.$zoomLevel.removeClass("hidden");
+    } else if (this.isTooBig) {
+      this.$image.addClass("cursor-zoom-in");
+    }
+  }
+
+  get zoomLevel() {
+    return this.$image.width() / Number(this.$image.attr("width"));
+  }
+
+  get isDownscaled() {
+    return this.$image.width() < Number(this.$image.attr("width"));
+  }
+
+  get isTooBig() {
+    return this.$image.width() > this.$component.width();
+  }
+
+  get $image() {
+    return this.$component.find(".media-asset-image");
+  }
+
+  get $zoomLevel() {
+    return this.$component.find(".media-asset-zoom-level");
+  }
+}
+
+$(MediaAssetComponent.initialize);