Webpack (#1367)

* Replace gulp with webpack and npm scripts for JS compilation
* Set up Travis CI to commit compiled JS
* Restructure `js` directory; only one instance of npm, forum/admin are "submodules"
* Refactor JS initializers into Application subclasses
* Maintain partial compatibility API (importing from absolute paths) for extensions
* Remove minification responsibility from PHP asset compiler
* Restructure `less` directory

less/common/common.less

@@ -0,0 +1,31 @@
+@import "fontawesome";
+@import "fa-brands";
+@import "fa-regular";
+@import "fa-solid";
+@fa-font-path: "./fonts";
+
+@import "normalize";
+@import "print";
+@import "scaffolding";
+@import "sideNav";
+@import "mixins";
+
+@import "App";
+@import "Alert";
+@import "AlertManager";
+@import "Avatar";
+@import "Badge";
+@import "Button";
+@import "Checkbox";
+@import "Dropdown";
+@import "Form";
+@import "FormControl";
+@import "LoadingIndicator";
+@import "Modal";
+@import "Navigation";
+@import "Placeholder";
+@import "Search";
+@import "Select";
+@import "Tooltip";
+
+@import "variables";