feat: add hide/close DM support (Slack-style DM management) (#157)

Co-authored-by: Tyler Longwell <tlongwell@squareup.com>

schema/schema.sql

@@ -57,6 +57,7 @@ CREATE TABLE channel_members (
     invited_by  BYTEA,
     removed_at  TIMESTAMPTZ,
     removed_by  BYTEA,
+    hidden_at   TIMESTAMPTZ,
     PRIMARY KEY (channel_id, pubkey)
 );