Upgrade from 1.x-dev: the 2.0 mail templates aren't used

The last Subscriptions 1.x-dev had one single mail template (1.9 had none). I've been trying to carry over a possible customization of the 1.x-dev template into 2.0, and this has been the cause for a long running issue: #181334: Problem with mail @variables after upgrading from 1.9.

The current Subscriptions 2.1 converts the (1.x-dev) @variables into (2.x) !variables, so that they work correctly, but it finally dawns on me that this was a bad idea all along. Since 2.x offers an array of specialized templates with many more variables, I didn't want to clobber the 2.x default templates with the 1.x-dev customization, but I used the latter if it was there.

Unfortunately, this has the effect that there's no good way to get rid of the 1.x-dev customization and to activate the 2.x templates (even though you can edit them, they just won't be used!). I'm not yet sure how to address the problem, but I'll come up with a fix shortly.

For an immediate solution, you can either disable, uninstall, and reinstall Subscriptions 2.1, which will remove the old 1.x-dev custom template variables (and also all of your Subscriptions customizations and user data!!!), or you can go into the {variable} table in your database and delete the 'subscriptions_email_subject' and 'subscriptions_email_body' variables.