A11y concerns and semantics

[binyamin]

Fixes https://github.com/marksuth/indigenous-desktop/issues/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

• [x] sidebar element doesn't exist. Probably meant to be aside.
• [ ] <a> should be <button>, since it doesn't actually open a new URL.
  • [x] "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).
• [x] The title attribute is not ideal for screen-readers.