Add logic concerning a reading's normal form when there is a lemma set

[tla]

The term 'lemma' is annoyingly overloaded, but...

When a reading is chosen as a lemma, its spelling and orthographic (and arguably punctuation) variants should have the same normal form as the lemma.

[tla]

First part of solution in commits 45095be and eef2a5a (when a lemma is set and when a relationship is made); also need to handle case when normal form of a lemma with relationships is changed.