- plain implementation (no ensures)
- simple test

Add virtual workspace Cargo.toml for the proposals module with three modules:
- engine
- codex
- discussion

- add update_post() extrinsic with tests
- rename create_discussion() to the create_thread()

- change MaxPostEditionNumber from config() to Get<> parameter in the discussion proposal module

- add ‘title is empty’ check
- add ‘title is too big’ check

Add ensure_* for:
- add_post
- update_post

# Conflicts:
#	modules/proposals/codex/src/lib.rs
#	modules/proposals/engine/src/tests/mod.rs