[FR] Add Feature flags (#4982)

* make currency choices independend

* Remove check for field, just try to get rid of it

* Add IF EXISTS to avoid error (works in postgres)

* Look for operational error, not programming error

* Use variants, depending on errors caused

* [FR] Add Feature flags
Fixes #4965

* Add option to define custom flags

* Revert "make currency choices independend"

This reverts commit ab84a7ff83.

* try fixing mysql

* more safeguards

* fix executioner call

* a fck

* use migrations. syntax

* and another round for mysql

* revert print change

* use UTC for datetime

* Update part.migrations.0112

- Add custom migration class which handles errors

* Add unit test for migration

- Ensure that the new fields are added to the model

* Update reference to PR

* fix ruleset for missing_models

* fix ruleset for flags_flagstate

* add API endpoints for flags

* add tests for new API endpoints

* fix tests

* fix merge

* fix tests

---------

Co-authored-by: martin <martin@iggland.com>
Co-authored-by: Oliver Walters <oliver.henry.walters@gmail.com>

InvenTree/config_template.yaml

@@ -228,3 +228,11 @@ remote_login_header: HTTP_REMOTE_USER
 #   splash: splash_screen.jpg
 #   hide_admin_link: true
 #   hide_password_reset: true
+
+# Custom flags
+# InvenTree uses django-flags; read more in their docs at https://cfpb.github.io/django-flags/conditions/
+# Use environment variable INVENTREE_FLAGS or the settings below
+# flags:
+#  MY_FLAG:
+#     - condition: 'parameter'
+#       value: 'my_flag_param1'