Use multiple tabs in dev setup

[klappradla]

When opening Firefox/Chrome with locally running versions of the extension, we could instruct the browsers to open multiple taps instead of just GitHub.

We could open tabs for all trackers where we have publicly hosted examples for.

Originally posted by @andreasknoepfle in https://github.com/bitcrowd/tickety-tick/pull/456#discussion_r2432241362