Enrolling/removing members from a community (#297)

* change(runtime/kreivo): add pallet-communities as dep

* change(pallets/communities): set sp-runtime as dep to support StaticLookup

* feat(pallet/communities): checks for community state

* feat(pallets/communities): check for permissioned calls

* feat(pallets/communities): implement add_member

* change(pallets/communities): use try_mutate_exists on do_insert_member

* chore(community:tests): remove redundant submod

* feat(pallets/communities): implement remove_member

* change(pallets/communities): update documentation

* change(communities:Cargo): update package metadata

* fix(pallets/communities): apply clippy suggestions

* change(pallet/communities): update docs

* fix(communities:remove_member): make sure that origin is being checked on first

* chore(pallets/communities): explicit naming for functions that ensure origin