Avoid func declarations using `function` keyword (#2813)

We had only a few function expressions declared with the function keyword; almost everything is using function declarations.

This came up after this discussion: https://github.com/badges/shields/pull/2803#discussion_r249011621 though is actually not related ot that example.

What’s being removed is a third option, which is assigning to a variable a function expression using the `function` keyword. There’s still room for the programmer to choose between arrow function expressions and function declarations.