Low latency fallback

[mjholub]

I would like to introduce the ability to make the extension use the fastest instance as the fallback if the user sets the latency threshold to a too low value (i.e. there are no instances for which the latency is lower than the set value), please pardon the confusion from erroneous branch name. Issues that need addressing:

1. The switch does not show up in the dark mode.
2. It is located at the bottom of the settings screen, while it should be right below the latency slider.