Use XcodeGen files for app variants and setup Enterprise submodule. (#2976)

- Create an app.yml with the base app variant.
- Override app.yml contents on Nightly/PR builds.
- Add AppHooks mechanism.
- Add Enterprise submodule.
- Conditionally import Enterprise/fastlane/Fastfile if available.

fastlane/README.md

@@ -45,14 +45,6 @@ For _fastlane_ installation instructions, see [Installing _fastlane_](https://do
 
 
 
-### config_nightly
-
-```sh
-[bundle exec] fastlane config_nightly
-```
-
-
-
 ### config_production
 
 ```sh
@@ -61,6 +53,14 @@ For _fastlane_ installation instructions, see [Installing _fastlane_](https://do
 
 
 
+### config_nightly
+
+```sh
+[bundle exec] fastlane config_nightly
+```
+
+
+
 ### upload_dsyms_to_sentry
 
 ```sh