Extension and profile handling refactored

- fixed Chrome popup on local browser "--ignore-certificate-errors" (not shown anymore)
- introduced methods "get_browser_profile" and "add_extension_to_browser"
- reduced code duplicities + increased flexibility when dealing with browser profile (ChromeOptions or FirefoxProfile)
Nit: do we need a `return_value` variable? Wouldn't it be simpler to just `return false` and `return true`?