frontend: use gulp, eslint, code refactor

Apologies in advance for the insanely huge commit. This commit is
primarily based on Anton Linevych's amazing work found here [1]. While
he had gone through the pains of making small, atomic changes to each
file, I had put off reviewing the PR for a long time. By the time I
finally got around to it, the project had changed so much that it didn't
make sense to keep the commits the same way. So I've cherry-picked most
of his commits, with some changes here and there, and I've squashed them
into one commit.

[1] https://gitlab.com/linevych/commento-ce/tree/feature/frontend_building_improvements

frontend/.eslintrc

@@ -0,0 +1,33 @@
+{
+  "env": {
+    "browser": true
+  },
+  "globals": {
+    "$": true
+  },
+  "rules": {
+    "no-bitwise": 2,
+    "camelcase": 2,
+    "brace-style": ["error", "1tbs"],
+    "curly": ["error", "all"],
+    "eqeqeq": ["error", "smart"],
+    "indent": ["error", 2],
+    "no-use-before-define": [
+      2,
+      {
+        "functions": false
+      }
+    ],
+    "new-cap": 2,
+    "no-caller": 2,
+    "quotes": [
+      2,
+      "double"
+    ],
+    "no-unused-vars": 2,
+    "strict": [
+      2,
+      "function"
+    ]
+  }
+}