A11y concerns and semantics

[binyamin]

Fixes #149

Summary

• Rewrites the markup, to create more semantic HTML and improve accessibility
• When markup breaks the CSS, this PR updates the CSS.

To-Do

• sidebar element doesn't exist. Probably meant to be aside.
• <a> should be <button>, since it doesn't actually open a new URL.
  • "menu" section
  • "channels" section
• Right now, Every single heading element on the page is an <h3>.
  • The heading-levels should be different, depending on the content. Like an outline.
  • There should always be a top-level heading (h1).
• The title attribute is not ideal for screen-readers.