Highlight and explain MRCA tests in tree popup.
Highlight and explain MRCA tests in tree popup.

Explain the intent of these tests and how they can help avoid wayward
OTU mappings. Addresses #767.

OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree