Addition Polish, Fix Accessibility, Remove Twitter Reference

[tfreebern2]

• Change <h4> tags to <h1> in About section to resolve accessibility issue
• Wrapped the <a> tag around the "Read More" in the History box so that the aria-label correctly matched the text link
• Remove "Follow us on Twitter" in blog sidebar
• Updated colors for "Join zCore" and "Learn More" buttons so that they pass accessibility for both light and dark backgrounds
• Fixed text color for the Careers > Our Team title. It was white on a white background. Also fixed the margin.
• Used this tool provided by Lighthouse to determine the correct contrast ratio in order to pass the scan https://dequeuniversity.com/rules/axe/4.8/color-contrast