Close the addthis relatedposts div, and only include it if addthis is enabled

[olipratt]

Hi,

Because this I hope is just a really minor fix, I've just opened a PR with the fix rather than open an issue - but if you'd prefer discussion in an issue first let me know and I'll raise one.

I just took an updated version of this theme, and noted that an addthis div has appeared on all articles despite me not using that feature, and also that the div wasn't closed.

This small fix just closes the div (assuming it's supposed to be closed), and only adds it if ADD_THIS_ID is set, to match the block of configuration a few lines above.

Also, I built the test pages and checked this worked correctly both with and without ADD_THIS_ID set.

Thanks again for creating and maintaining this theme!

[alexandrevicenzi]

Thanks for this PR. It shouldn't be added if you don't use Add This.