Multi-language posting

Resolves #2975

• AP: accept *Map
• MastoAPI: renders
  • Status
  • history
  • source
  • Attachment
  • Poll
  • render language attr for posts with specified lang, multiple lang using mul
• MastoAPI: posting
  • status
  • poll
  • attachment
  • validate language tags
  • support language attribute in posting
• MRF?
  • [n/a] activity_expiration_policy.ex
  • [n/a] anti_followbot_policy.ex
  • [n/a] anti_link_spam_policy.ex -> doesn't change any content
  • [n/a] drop_policy.ex
  • ensure_re_prepended.ex
  • [n/a] follow_bot_policy.ex
  • [n/a] force_bot_unlisted_policy.ex
  • force_mentions_in_content.ex
  • [n/a] hashtag_policy.ex
  • [n/a] hellthread_policy.ex
  • keyword_policy.ex
  • [n/a] media_proxy_warming_policy.ex
  • [n/a] mention_policy.ex
  • no_empty_policy.ex -> will crash
  • [n/a] no_op_policy.ex
  • no_placeholder_text_policy.ex
  • normalize_markup.ex
  • [n/a] object_age_policy.ex
  • [n/a] reject_non_public.ex
  • [n/a] simple_policy.ex
  • [n/a] steal_emoji_policy.ex
  • [n/a] subchain_policy.ex
  • [n/a] tag_policy.ex
  • [n/a] user_allow_list_policy.ex
  • [n/a] vocabulary_policy.ex
• Filtering is VERY hard. I'm not going to implement it in this mr. Prepare for filtering: inject language attribute language = '[{"lang": "<bcp47 subtag>", "variants": ["<bcp47 subtag>"]}]' into object, index with GIN jsonb_path_ops ( https://www.postgresql.org/docs/current/datatype-json.html#JSON-INDEXING ) so later one can use language @> '[{"lang":"xx", "variants": ["aa", "bb"]}]' to get only posts containing one language (how to get posts that contains at least one of the languages specified?)