[WIP] Experimental use webpack - #1997

I tried to introduce webpack. This PR contains the result.

advantage to current implementation

• resource bundling

Now, many resources are included from every page. Webpack can bundle these resources.

before:

after:

Of cource, these resources are cached by browser normally.

• minification

In webpack and sbt-webpack, minify js/css is simply done by webpack prod on sbt.

advantage to Webjars (#1595)

• Webjars is hard-tied to sbt and node.js

Rather, I think that it is appropriate to say that "Webjars is overkill". It imports npm dependency to jars one, so more lines required to write in build.sbt.

Webjars also has more complicated dependency by webjars classic/npm/bower jars. At least I know, trouble will occur when using mixed them.

• management library

webpack uses simple npm's package.json. It can use GitHub's repo directory. So, we can manage our customized jsdifflib as gitbucket/jsdifflib.

• simplicity

If we needs new javascript library, adding it is very easy. npm install --save and require('library'); to main.js.

disadvantage

• need node.js/npm install for developer

User doesn't need node/npm. Simply run java -jar gitbucket.war. But for building GitBucket, npm install downloads node_modules and sbt-webpack requires node.js.

TODOs

I didn't check all functionality behavior. At least, diff doesn't work yet. It is required to do modulerize our javascript files.