Revisit UI design once we have float and table basic support

[atbrakhi]

We are advancing layout2020. So upcoming months we will have more support hence more opportunity for demos.

Idea is to revisit the UI design so that we can simplify it.

• Implement a proper header nav. for example we can create a header with servo logo and add contribute button on header
  • Implement a proper header #20
• Next for the body part where we show experiments, create a drop down to show all the experiment features(like webGL, Rendering etc) in the dropdown, so that user can select and see the experiment that interests them.

Note: We need to make sure all the changes working properly when we run demos.servo.org on servo.

I am just dumping some ideas here. Feel free to ask for clarification if needed or comment if you have suggestion