feat: add new required slug property to coder_app, use in URLs (#4573)

coderd/database/migrations/000066_app_slug.up.sql

@@ -0,0 +1,16 @@
+BEGIN;
+
+-- add "slug" column to "workspace_apps" table
+ALTER TABLE "workspace_apps" ADD COLUMN "slug" text DEFAULT '';
+
+-- copy the "name" column for each workspace app to the "slug" column
+UPDATE "workspace_apps" SET "slug" = "name";
+
+-- make "slug" column not nullable and remove default
+ALTER TABLE "workspace_apps" ALTER COLUMN "slug" SET NOT NULL;
+ALTER TABLE "workspace_apps" ALTER COLUMN "slug" DROP DEFAULT;
+
+-- add unique index on "slug" column
+ALTER TABLE "workspace_apps" ADD CONSTRAINT "workspace_apps_agent_id_slug_idx" UNIQUE ("agent_id", "slug");
+
+COMMIT;