Make sidebar sticky

[milanmlft]

Keeps the sidebar navigation menu visible when scrolling past it.
I mainly implemented this as a request for https://github.com/learntodiscover/varnish but thought I'd contribute it here as well. Happy to address any feedback!

Fixes #16

[ErinBecker]

Thank you for this contribution @milanmlft! I'll take a look at it with @froggleston at one of our upcoming co-working meetings.

[milanmlft]

Hi @ErinBecker! Hope you're doing well. I know you guys are super busy, but did you have a chance to look at this yet? Thanks!

[ErinBecker]

Hi @milanmlft - Happy New Year! 🥳 I'm back at my desk with renewed percent effort to dedicate to Workbench and am finally reviewing your contribution. Thank you so much for your patience.

I've locally tested this PR and can confirm that the sidebar is indeed sticky! I personally agree that this is a great improvement to the lesson formatting and makes navigation much easier. After reading through some of the conversation on #16 and further issues referenced there, I want to do my due diligence and make sure this solution is compatible with some of the concerns raised there:

1. @zkamvar raised a concern with the heaviness of adding additional JavaScript libraries. I have zero experience with JS and don't know how to test how much weight is added with this implementation. Any thoughts you have would be appreciated.

2. In October, @froggleston implemented sticky sidebar for glossario using https://github.com/abouolia/sticky-sidebar. I'd like to hear @froggleston's thoughts on whether we should use the same implementation for Workbench as Glossario for consistency.

3. On the Glossario PR referenced above, @Imran-imtiaz48 provided some accessibility feedback. I'm not clear whether this feedback is related to our implementation of sticky-sidebar, or if it is feedback on how sticky-sidebar itself works. If the later, we should perhaps consider using @milanmlft's implementation rather than sticky-sidebar.

Apologies for the inconclusive response after such a long delay. I'm back in the saddle and ready to start making progress towards implementing this improvement and will be following the conversation related to my questions above closely. @froggleston - please feel free to chime in.

[zkamvar]

@zkamvar raised a concern with the heaviness of adding additional JavaScript libraries.

FWIW, I don't think there are any new JS libraries in here, so that concern is taken care of.