Keep lemma translation for every form

Closes #478

This was not a bug per se - in fact it did exactly what I wanted it to
do, but probably under a false assumption. Every form stored their own
lemma translation, but as there is no visual link between them, this
appears to be counterintuitive.
Trigger a need for a save when gloss is updated
Translate Lemma Translation label
Fix french translation
Merge pull request #496 from latin-language-toolkit/lemma_trans_field

Keep lemma translation for every form