[Backend] Add the FilesController

[zergov]

Part of https://github.com/visevol/GithubVisualisation/issues/41

Add a FilesController.

This controller returns information about a file on a repository. Right now, it only returns the following information:

• filepath
• commits count
• main contributor

Example:

GET /repositories/123/files/head/activejob/lib/active_job.rb

{
  filepath: "activejob/lib/active_job.rb",
  commits_count: 75,
  main_contributor: {
    author: "Ryuta Kamizono",
    commits_count: 7
  }
}