Improve linear gradient component

[mikelbrierly]

User Story:

As a developer, I want to be able to use the linear gradient component in more situations, So that we don't duplicate our code unnecessarily

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The linear gradient component currently only supports a horizontal format, but we can extend it to be usable in either a horizontal or in a vertical context

Acceptance Criteria:

• [x] Update LinearGradient.js to accept a height prop as well as width, and use it to set a height css value just like we're doing with the width.
• [x] Adjust the css so that the line component has a width property that can be fell back on if one isn't provided in the component props
• [x] To test, use the LinearGradient component, and pass in height for props instead of width. You should see a vertical line now!

[aggarwalsshreya]

I tested the code, but it seems like the code overrides the width and height that I put into the component and takes in the width and height from the CSS file first.