Update the underline implementation

[mansona]

Right now we're using background to give us a custom underline implementation, but it looks like CSS has come a long way since we implemented this

We should seriously consider using more modern techniques such as this youtube short https://www.youtube.com/watch?v=9WarmW1S10U

[taiwo2]

@mansona Can you please assign this issue to me

[mansona]

@taiwo2 sorry it's not possible to assign issues to people not in the ember-learn org, but if you want to try implementing it then feel free 👍

[taiwo2]

@mansona kindly review my PR#524