make http links more obvious to highlight their insecurity

[alexwykoff]

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[bsclifton]

This functionality is really well done by the moarTLS extension by @ericlaw1979

Maybe we can offer this extension an option under advanced settings (similar to pocket)

cc: @bradleyrichter

[cndouglas]

Related: #1576 ("Block all HTTP" mode)

[tildelowengrimm]

Perhaps it's worth thinking about HTTP/HTTPS link styling at some point down the line. brave/brave-browser#866