Explain default themes better #385
The blue notices are new:
Closes #299. Other solutions are more complicated with lots of edge cases (even more edge cases).
Note, there are three places where this notice needs to be rendered:
Also, the new notice should only appear if both the child theme and the parent theme for a given year are present.
Some discussion at https://classicpress.slack.com/archives/CCCF22HG8/p1551283310028800