Add 'language' field to TextVersion

[spookylukey]

Created by: spookylukey

For future enhancements.

All current texts are English.

We might also consider adding a language variant, to distinguish old from modern English, for example, which could be useful in some circumstances e.g. automatically finding a corpus to use as a source of word suggestions.

[spookylukey]

Created by: spookylukey

The language model should itself have:

• a default TextVersion for selecting automatically if a person selects their language
• a default TextVersion to use for generating word suggestions.

Both must be nullable because of the dependency problem.

[spookylukey]

Created by: spookylukey

Implemented now, in most basic form.