[DB] rewrite migrations of all tables (25) + modification schema

db/migrations/20181110191614_create_groups.js

...10191757_create_associativetable_metagroup_memberships.js

db/migrations/20190510220159_create_users.js

db/migrations/20190510221450_create_groups.js

db/migrations/20190510230012_create_metagroups.js

db/migrations/20190510231558_create_simplegroups.js

...grations/20190510232551_create_simple_meta_memberships.js

db/migrations/20190511000700_create_messages.js

db/migrations/20190511000800_create_messages_events.js

...igrations/20190511000809_create_messages_announcements.js

db/migrations/20190511001555_create_messages_privateposts.js

db/migrations/20190511002946_create_messages_questions.js

db/migrations/20190511004131_create_messages_answers.js

db/migrations/20190511004811_create_announcements_authors.js

...rations/20190511011224_create_announcements_recipients.js

db/migrations/20190511011333_create_events_authors.js

db/migrations/20190511011710_create_events_recipients.js

...ions/20190511011846_create_events_participating_groups.js

...tions/20190511012040_create_events_participating_users.js

db/migrations/20190511013800_create_requests.js