Flip the toggle on social media blocking options to address user confusion

[karenkliu]

Description

Currently, our social media blocking options in settings look like:

We need to reverse the order so that toggles are OFF by default and security-minded users must toggle the options ON to break sites. This is similar to how block scripts is "OFF" by default in Shields and users must turn it "ON" to break sites.

While making this fix, add Youtube and change the section label to "Social Media Blocking".

Designs

[rebron]

cc: @petemill