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/components/source_data_component/source_data_component.js

@@ -1,22 +0,0 @@
-class SourceDataComponent {
-  static initialize() {
-    $(document).on("click.danbooru", ".source-data-fetch", SourceDataComponent.fetchData);
-  }
-
-  static async fetchData(e) {
-    let url = $("#post_source").val();
-    let ref = $("#post_referer_url").val();
-
-    e.preventDefault();
-
-    if (/^https?:\/\//.test(url)) {
-      $(".source-data").addClass("loading");
-      await $.get("/source.js", { url: url, ref: ref });
-      $(".source-data").removeClass("loading");
-    }
-  }
-}
-
-$(document).ready(SourceDataComponent.initialize);
-
-export default SourceDataComponent;