How to do plugins

[hmpf]

I'm not happy with how our plugins are looking. We currently have one type, notification plugins, and soon will have issue system plugins as well. The plan is to support only one issue plugin at a time, but we already support multiple notification plugins.

Loading plugins

One plugin of a specific type at a time

Loading an issue plugin via dotted path is fine, since we plan to support only one at a time.

Many plugins of a specific type at a time

Loading and using multiple notification plugins at the same time can be done in many ways:

1. We currently do it via a handcrafted setting, a list of dotted paths
2. We could switch to django apps, but then the dotted path still have to be added manually, to INSTALLED_APPS.
3. Magical things to do with with .pth-files. No thankee.
4. Some magical plugin framework. Which means yet another dependency.
5. Namespace packages are a pain. Let's just not.
6. We could switch to using entry points (see for instance https://setuptools.pypa.io/en/latest/userguide/entry_point.html#entry-points-for-plugins). It's just a matter of installing the plugin, no need for changing a setting in addition.

If we do something where we do not manually type in what plugins we want via dotted path (like in 3, 4, 5, 6) what do we do with the email-plugin and sms2email-plugin?

Writing plugins

Having an abstract class with static methods is fugly. If there are only static methods, why not import the module, not the class, and have functions instead?

[hmpf]

The email-plugin and sms2email-plugin are different because they (currently) depend on site-wide settings taken from settings.py. The other notification plugins have destination-specific settings.