[DSL] Migrate in some generic helper functions

[orta]

From jestjs/jest#2508

danger.github.linkFiles ->

// Takes a list of file paths, and converts it into clickable links
const linkableFiles = (paths: Array<string>): string => {
  const repoURL = danger.github.pr.head.repo.html_url;
  const ref = danger.github.pr.head.ref;
  const links = paths.map(path => {
    return createLink(`${repoURL}/blob/${ref}/${path}`, path);
  });
  return toSentence(links);
};

I think we probably also need util functions that are specific to universal Danger use-cases, mainly as the JS core lib is relatively weak:

danger.utils.sentence:

// ["1", "2", "3"] to "1, 2 and 3"
const toSentence = (array: Array<string>) : string => {
  if (array.length === 1) { return array[0]; }
  return array.slice(0, array.length - 1).join(', ') + ' and ' + array.pop();
};

danger.utils.href:

// ("/href/thing", "name") to "<a href="/href/thing">name</a>"
const createLink = (href: string, text: string): string => 
  `<a href='${href}'>${text}</a>`;

[orta]

These are in 👍