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

.travis/build.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+
+main() {
+  while getopts ":k:i:" opt; do
+    case $opt in
+      k) encrypted_key="$OPTARG"
+      ;;
+      i) encrypted_iv="$OPTARG"
+      ;;
+      \?) echo "Invalid option -$OPTARG" >&2
+      ;;
+    esac
+  done
+
+  cd js
+  npm i -g npm@6.1.0
+  npm ci
+  npm run build
+
+  git add dist/* -f
+  git commit -m "Bundled output for commit $TRAVIS_COMMIT [skip ci]"
+
+  eval `ssh-agent -s`
+  openssl aes-256-cbc -K $encrypted_key -iv $encrypted_iv -in ../.deploy.enc -d | ssh-add -
+
+  git config user.name "flarum-bot"
+  git config user.email "bot@flarum.org"
+
+  git push git@github.com:$TRAVIS_REPO_SLUG.git $TRAVIS_BRANCH
+}
+
+main "$@"