Make link for new github repo open in new tab

[yufengg]

Unfortunately GitHub markdown does not support opening links in a new tab. This change replaces the link to create a new github repo with some html that does that.

Here's a github issue thread that shows that markdown is not likely to support this natively in the future: mojombo/github-flavored-markdown#28

[ebidel]

We don't use github markdown. Clicking that link on https://www.polymer-project.org/docs/start/reusableelements.html works exactly as I'd expect.

[yufengg]

The trouble is that it opens it in the same window, and thus wipes out the
progress made on the page. One can make an argument for opening it in a new
tab being a better experience.

[ebidel]

I'm fairly certain all external links on the site open in the same tab. That's typically what users expect. If they want a new tab, they can use the ctrl/cmd key.

[yufengg]

Fair enough. Just thought it was odd compared to other websites' linking
standards. At least now I know what to expect too :)

[addyosmani]

Reviewing, this appears to work as expected and we don't intend on changing the current behaviour. Closing. If there's still interest in us doing this feel free to reopen.

[Gribbs]

Opening link in a new tab should be an option. It's useability 101. Users should not have to lose their current page if the page designer doesn't want it to work like that (like me right now).