Allow article sidebar to be sticky

[joshdarby]

Changes

This pull request makes the following changes:

• Wraps the single two column template content and sidebar in a parent div named content-container, which makes sure the sidebar is the same height as the content so that position: sticky; works on the sidebar.
• Updates get_the_ID to get_the_ID() in the content-single.php partial so there's no ugly undefined constant error

Note that we should probably warn them that they'll need/want to remove some widgets from the sidebar now. position:sticky; doesn't allow you to scroll in the sidebar so you either wait until you're scrolled 100% down the page, or you make the sidebar height less than the viewport height by reducing the number of widgets

Currently with a ton of widgets:

With only 2 widgets:

Why

For #52

Testing/Questions

Features that this PR affects:

• Single two column template and sidebar display

Questions that need to be answered before merging:

• [ ] Is this PR targeting the correct branch in this repository?
• [ ] Is anything broken in here?
• [ ] Is it possible to add overflow: scroll; to the sidebar? Every way that I have tried has broken the sticky positioning

Steps to test this PR:

1. View a post with a sidebar
2. Scroll, scroll, scroll and make sure the sidebar stays sticky!