collab: Allow enabling feature flags for all users (#16372)

This PR adds a new `enabled_for_all` column to the `feature_flags` table
to allow enabling a feature flag for all users.

Release Notes:

- N/A

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

@@ -295,7 +295,8 @@ CREATE UNIQUE INDEX "index_channel_buffer_collaborators_on_channel_id_connection
 
 CREATE TABLE "feature_flags" (
     "id" INTEGER PRIMARY KEY AUTOINCREMENT,
-    "flag" TEXT NOT NULL UNIQUE
+    "flag" TEXT NOT NULL UNIQUE,
+    "enabled_for_all" BOOLEAN NOT NULL DEFAULT false
 );
 
 CREATE INDEX "index_feature_flags" ON "feature_flags" ("id");