Improve code consistency and clarity

[MarkBiesheuvel]

This pull request does the following things:

• Replace all Windows line endings \r\n by Unix line endings \n. (See this line if you still want Windows line enings locally)
• Replace all tabs by spaces, as it was a mixture of both before.
• Remove trailing whitespace at end of line
• Define a code style with eslint
• Replaces css styling on id by styling on class names
• Replace custom HTML generation by jQuery functions
• Replace recursive calls of assumeRole by Promises

I understand that this is a big overhaul, so I tried to not change any functionality. It is probably best to review it commit by commit.

Hopefully you find this suggestion useful.