Cycle to other mirrors when available

[jheretic]

This PR addresses #16, displaying a green icon when the user visits a site which is a mirror. This also integrates #20, so that Tor mirrors are supported as well. Targeted behavior for testing:

• As before, if visiting a site with a mirror available, the button turns red and when clicked offers a button to go to a mirror.
• Additionally, if there is a Tor mirror, the button turns purple. The popup includes a message about installing Tor as well as a button to open a new tab to go to the Tor browser install page, and a button to copy the Tor URL to the clipboard.
• If on the page for a standard mirror, the button turns green. If this is the only mirror, the popup includes a message to that effect and no button. If there are other mirrors available, the popup includes a corresponding message and a button to cycle to a new mirror, not including the current mirror. Note that visited mirrors are not preserved, so subsequent clicks of the button could cycle back to a previous mirror.
• Whenever a Tor mirror is available, the Tor message and buttons are shown in addition to other messages above, with the exception of when the user is on a standard mirror and the only other available mirror is a Tor one, in which case the message about this being the only available mirror is suppressed. The purple color of the button supersedes the red or green color above.

[harumhelmy]

Hmm! So I ran npm run build && npm run start:firefox. I'm not seeing the color changes on the extension button, though I am seeing the options in the popup as you say. Am I missing a step in testing?

[irl]

Should we assume that torproject.org is blocked? It might be an idea to link to https://tor.eff.org/download/ instead.