Improve the webextension design

In comment:40:ticket:23888 @antonela wrote,

Some comments:

• The icon is a great way to offer visual feedback to users. I remember we discussed it a bit at https://trac.torproject.org/projects/tor/ticket/27385#comment:8. Using the states described at the comment:35:ticket:23888, we have:

1. Gray icon for disabled
2. Purple icon for enabled (waiting)
3. Rotating purple icon for proxying (enabled + transferring). I did the animation with CSS here ​https://snowfl4k3.glitch.me/.

• The doorhanger can be narrower. 320px is fair enough.
• Could we use the system font? In that way, it will use the client's default. Now, it is loading Times New Roman.
• The copy is great. Although, I think we can iterate over the technical terms in the future.