Make link state colors more different

[sterndata]

Currently, the standard link color for a topic is #0073aa, with the visited color being #0073aa. Yup -- the same.

A forum suggestion was to make it easier to see what you've already looked out, so how about changing the visited link color to something a grayer blue? #93acbb perhaps.

[netweb]

A couple of mockup screenshots would be great @sterndata if you could

Ideally using colors from the WordPress Color Palette would be ideal

​https://make.wordpress.org/design/handbook/design-guide/foundations/colors/

[SergeyBiryukov]

One concern here is that we no longer have ?replies=X appended to topic URLs, like bbPress 1.x used to do, so if the topic has new replies, the color alone won't indicate that.

Going to close as a duplicate of #2569, since it's essentially the same issue.