Replace the Muli font with Poppins

[petemill]

We have decided to stop using Muli, which we mainly used for body content. Uses should be replaced with Poppins. When replacing, we generally have to dial down the font-weight a little (e.g. from 600 to 500).

This requires changes in brave-core and brave-ui (where there are theme variables and shared styles from components).

Places:

• Welcome
• Rewards (page, not panel)
• NTP widgets

Test Plan

• Open brave://welcome
• Verify that the font being used is Poppins not Muli, via referencing the below screenshots
• Turn on and open brave://rewards. Verify the font (for the "left" column) is correct as per the below screenshots
• Open NTP and switch to Brave Talk widget. Verify screenshots...

Screenshots (old on right, new on left)

Notes

If you really want to verify which font is being used, you can inspect the element, switch to the "computed" tab and scroll right to the bottom. DevTools will report which font is being used to actually render.