Select default gas price setting

Description:
Add a setting to choose a default gas track among the (Slow / Normal / Rushed). Right now we need to change manually for each transaction and it always take the Normal track as default. For the past weeks/months gas price has been low enough that the Normal and Slow tracks are equivalent almost always, but users pay a 2-3x premium because of this setting.

Purpose:
A better user experience in choosing gas and less user abuse when gas price is low.

Extension/Mobile/Both:
Both

Images/Attachments:

Hi and thank you for sharing this feature request! :grinning_face: :coffee: