Extracted links into sidebar partial, changed some indentations.

Hi Catherine
I've added a few changes to the blog, I'm going to add a bunch of comments below to explain why i'm making the changes, so hopefully it will be helpful to you. You don't have to merge this PR, feel free to reject it and delete the branch so that you don't end up with a bunch of my commits throughout your repo.

The main changes are:
* Moving the sidebar into its own partial.
* Formatting of the posts controller and views.
* Linking to tagged posts from the sidebar.