Removes trailing whitespace from file-header

[cwgorman]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Overview of change:

The rule file-header does not play nice with no-trailing-whitespace if the file header includes blank lines. For example, after applying file-header with --fix, you might end up with something like this:

/* 
 * Copyright My Company 2018
 * 
 * Project Identifier
 * 
 * All Rights Reserved, blah blah blah legal stuff.
 */

Note the trailing whitespace on the * lines. This means you would have to run tslint --fix twice to get the desired result.

This PR adds a simple solution of right-trimming each line of the file header.

Is there anything you'd like reviewers to focus on?

CHANGELOG.md entry:

[bugfix] Makes rule file-header play nice with rule no-trailing-whitespace

[palantirtech]

Thanks for your interest in palantir/tslint, @cwgorman! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[suchanlee]

Thanks @cwgorman!