Add a role column to the database and start using it

We cannot yet stop using `admin` because stable will continue writing
it.

crates/collab/migrations.sqlite/20221109000000_test_schema.sql

@@ -226,6 +226,7 @@ CREATE TABLE "channel_members" (
     "channel_id" INTEGER NOT NULL REFERENCES channels (id) ON DELETE CASCADE,
     "user_id" INTEGER NOT NULL REFERENCES users (id) ON DELETE CASCADE,
     "admin" BOOLEAN NOT NULL DEFAULT false,
+    "role" VARCHAR,
     "accepted" BOOLEAN NOT NULL DEFAULT false,
     "updated_at" TIMESTAMP NOT NULL DEFAULT now
 );