Add requires_zed_cla column to channels table

Don't allow granting guests write access in a call where the channel
or one of its ancestors requires the zed CLA, until that guest has
signed the Zed CLA.

Co-authored-by: Marshall <marshall@zed.dev>

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

@@ -196,7 +196,8 @@ CREATE TABLE "channels" (
     "name" VARCHAR NOT NULL,
     "created_at" TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     "visibility" VARCHAR NOT NULL,
-    "parent_path" TEXT
+    "parent_path" TEXT,
+    "requires_zed_cla" BOOLEAN NOT NULL DEFAULT FALSE
 );
 
 CREATE INDEX "index_channels_on_parent_path" ON "channels" ("parent_path");