Add site setting for category of topics created by embedding

This patch adds a site setting (under 'Embedding') for the category that is used for topics created when a blog post is retrieved when first embedding comments on that post. This is as suggested on a thread in meta:

https://meta.discourse.org/t/discourse-plugin-for-static-site-generators-like-jekyll-or-octopress/7965/106

I have only provided the English string localisation of the setting description.

Because of the difficulties of setting up a sandboxed development environment to test this, requiring both a Discourse server and a static blog server (e.g., Jekyll), I found it easier to test this rather small patch *in situ* on my production servers. It worked.