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/.gitignore

@@ -1 +1,2 @@
 .sass-cache
+node_modules/