Add `lang` column to `post` table (ISO 639-1 code)

[andreaskoepf]

Add a string(2) lang field to the post model class to specify the language of a given post. This is necessary to query posts in a given language (e.g. which the current user is able to speak).

Don't forget to add the alembic migration scripts.

[andreaskoepf]

We need to decide how we handle languages codes in the project, e.g. https://pypi.org/project/langcodes/ ?

[GraemeHarris]

@andreaskoepf Looking to pick up something small to get into the project - do you mind if I take this? Would we be looking to force a language on the BE API when creating tasks or leaving that as optional?

[yk]

@andreaskoepf Looking to pick up something small to get into the project - do you mind if I take this? Would we be looking to force a language on the BE API when creating tasks or leaving that as optional?

I think for now we can just default to english, eventually we could either force it or try to detect the language. I've assigned you

[GraemeHarris]

Have a PR up - should be simple enough :)