refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/plugins.js

@@ -0,0 +1,6 @@
+export { categoryCollapse } from './components/category-collapse';
+export { initClipboard } from './components/clipboard';
+export { imgExtra } from './components/img-extra';
+export { initLocaleDatetime } from './components/locale-datetime';
+export { initPageviews } from './components/pageviews';
+export { smoothScroll } from './components/smooth-scroll';