Add ability to inject HTML content into sidebar and sidebar-block partials

This MR resolves #249 (closed)

This feature adds the custom_sidebar front-matter variables. It is where the user will write their HTML content which will be displayed below the sidebar or sidebar-block.

I named the variable custom_sidebar to be similar to custom_jumbotron.

Navigate to /third-parent to see an example of this with a "Subscribe to Calendar" button.