Allow users to copy auth URLs

[IBBoard]

I've got multiple Twitter and Mastodon accounts (mine, Cawbird, Cawbird Test). I use a combination of Firefox containers (mine, Cawbird) and Chromium (Cawbird Test) to separate the authentication so that I don't keep logging in and out of accounts.

When Cawbird opens the auth link, it opens the default browser. Which is expected behaviour. But if I'm not logged in on the website then the browser redirects to the sign-in URL and I lose the original URL, so I can't copy it to my desired browser/container.

It would be useful to have a way to copy the authentication URL from the auth flow for when people use Twitter/Mastodon in non-default browsers or containers.

[IBBoard]

(I'm currently just logging it to the terminal so that I've got it there. But it probably gets us back to IBBoard/cawbird#274 if I merged it!)