Disable slow default extensions

[piercefreeman]

Default add-ons are opt-in because they are a page-dependent tradeoff. uBlock
takes extra processing time, but it can still be net faster on pages with lots
of ads or trackers. For SaaS sites with minimal or no ads, keep default add-ons
off so Rotunda does not spend time processing unnecessary extension rules.
Pass NewBrowser(playwright, default_addons=True) when that tradeoff is useful.

This change better aligns with stock firefox and the performance expectations of page load:

run                         first_field   post_resp_to_DCL   slowest_cached_resource
stock firefox               887.8ms       565.0ms            21.0ms
rotunda fixed default       852.5ms       561.0ms            22.0ms
rotunda default_addons=True 1731.5ms     1504.0ms          970.0ms