Refactored script to take an OOP structure

[kylekirkby]

This PR updates the check-links-3.py script to use an object-orientated structure to enhance readability/usability of the script. I've also added comments to the code in order to break down the functionality of the script for future contributors to the project.

I've built the container locally with these changes and all seems to work as expected!

@morancj @pcolmer

[pcolmer]

@kylekirkby have you tested it on a known broken site to confirm that internal and external broken links are being picked up?

[kylekirkby]

I've tested on latest Linaro.org develop branch locally with the built Docker container using a personal tag of "refactored".

It found some broken "external" links but I'll purposefully break internal some links and check now.

Should we add a "_broken_site" test directory at some point to do this upon the creation of a pull request to this repo?

[pcolmer]

Should we add a "_broken_site" test directory at some point to do this upon the creation of a pull request to this repo?

Probably not a bad idea but for it to work really well, we'd need to have a script that runs the container against the test directory and checks the output to make sure it is as expected. Since the external links are checked asynchronously, the progress characters can appear in a different order. Therefore this won't be a straightforward comparison to make. Something for the wishlist queue :)