Automate configuration of debusine client

We should have a script setup-debusine-client or similar that would simplify the creation of the initial configuration file for Debian developers (and other users).

It should:

1. Stop if ~/.config/debusine/client/config.ini already exists (or offer to replace it entirely?)
2. Ask for the debusine instance, defaulting to debusine.debian.net
3. Open the browser at $DEBUSINE_URL/-/me/token/ and invite the user to create a token (suggest a name based on the hostname and the date?), and prompt the user to copy/paste it in standard input
   • The assumption here is that the user will be redirected to the login page if he is not authenticated and will get back to the right page after login. If that's not true we should fix it.
   • NOTE: The above is currently not possible because the real token URL is like /-/user/$USERNAME/token/, so that's why I wrote /-/me/ as a possible smart redirect...
4. Create a valid ~/.config/debusine/client/config.ini
5. Run debusine list-work-requets
   • If it works, indicate success and finish.
   • If it fails, display the error, offer to retry by submitting another token, or to remove the configuration file and abort.