Establish modular CSS naming conventions

Summary
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To establish some CSS/SCSS conventions for Horizon.

Motivation
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Currently CSS is added to the project in an ad-hoc manner. It would be valuable to DRY up the CSS so our components can be more modular. This would allow us to take more advantage of the power of Sass and of cascading, particularly when creating custom themes.

As a first step towards the lofty goal of modularizing the architecture of our CSS, it is helpful to define what our CSS rules should look like. e.g. http://thesassway.com/advanced/modular-css-naming-conventions

Description
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This blueprint includes:

- Defining a set of naming conventions for our CSS
- Adding documentation/style guide for contributors and reviewers
- Creating some examples of these conventions in practice by refactoring some of the SCSS files in the root static folder
- Investigate if there is an appropriate CSS linter we can use

UX
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Shouldn't affect end user experience as this is simply adding documentation and some refactoring.

Testing
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- For the examples that get created, we will want to ensure we have adequate automated test coverage for those components, and should engage in a few manual tests. More info to come once we get there!

Outside Dependencies
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N/A

Requirements Update Required
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N/A

Doc Impact
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This blueprint is mainly about including some kind of documentation for contributors and reviewers to refer to.