Make more models depend on `ModelBase`
Our ModelBase provides a created and modified date field, as well as a few useful methods. The following models do not currently inherit from it:
ReindexingTranslationSequenceAddonCategoryAddonUserAddonUserPendingConfirmationFrozenAddonGroupUserApplicationsVersionsRssKeyConfig
Some of these would benefit from inheriting from ModelBase, especially as we might add more features to it in the future. At the very least for Config it would be really useful.
┆Issue is synchronized with this Jira Task
Note: when implementing this, make sure ESAddonSerializer passes created=None (or a more appropriate value coming from ES data) when creating new instances of these models, because of https://github.com/mozilla/addons/issues/9003.
Old Jira Ticket: https://mozilla-hub.atlassian.net/browse/ADDSRV-135