Language tags for communities/sites

[Nutomic]

• [ ] allowed languages are shown in site/community sidebar (user languages are not publicly shown)
• [ ] admins/mods can edit languages for sites/communities (using the same input as in user profile)
• [ ] during post/comment creation, language selection is limited to intersection between user languages and community languages. If that leaves only a single language, hide the input field (same logic as here)

see #771