Add ability to declare forms as optional

[lucaswerkmeister]

For some templates, it would be useful to be able to declare certain forms as optional, so that they would not be required even in non-advanced mode. Specifically:

• many Norwegian Bokmål adjectives have no comparative forms, according to @jhsoby (currently required in the template)
• Malayalam nouns have optional vocative case, according to User:Vis M (currently not part of the template)

This should be fairly straightforward to implement – just omit the required in the HTML, I think. The backend should already be able to handle missing forms.

[lucaswerkmeister]

Done in 77328e559d17d80845aad2f5a5c49f92633c694c (I forgot to mention the issue in the description).